Free T4 (FT4): free thyroxine, normal values and thyroid interpretation | Clinique Omicron<\/title>\n<meta name=\"description\" content=\"Free T4 (FT4) is the active fraction of thyroxine not bound to proteins. Low FT4 hypothyroidism, high FT4 hyperthyroidism, TSH FT4 FT3 combined interpretation and levothyroxine monitoring in Quebec.\">\n<meta name=\"keywords\" content=\"T4 libre, FT4, thyroxine libre, T4 libre interpr\u00e9tation, FT4 hypothyro\u00efdie, FT4 hyperthyro\u00efdie, TSH FT4 bilan thyro\u00efdien, FT4 valeurs normales, T4 libre suivi l\u00e9vothyroxine, T4 libre Qu\u00e9bec\">\n<link rel=\"preconnect\" href=\"https:\/\/fonts.googleapis.com\">\n<link href=\"https:\/\/fonts.googleapis.com\/css2?family=Cinzel:wght@600&family=Poppins:wght@400;500;600;700&display=swap\" rel=\"stylesheet\">\n<style>\n@import url('https:\/\/fonts.googleapis.com\/css2?family=Cinzel:wght@600&family=Poppins:wght@400;500;600;700&display=swap');\n\n.co-wrap * {\n font-family: 'Poppins', sans-serif;\n box-sizing: border-box;\n}\n.co-wrap {\n max-width: 1100px;\n margin: 0 auto;\n padding: 30px 0 60px;\n margin-top: 10px;\n}\n.co-label {\n font-family: 'Cinzel', serif;\n font-size: 14px;\n font-weight: bold;\n letter-spacing: 1px;\n text-transform: uppercase;\n color: #4D6577;\n margin-bottom: 14px;\n display: block;\n}\n.co-wrap h1 {\n font-size: 32px;\n font-weight: 500;\n color: #323C52;\n margin: 0 0 22px;\n line-height: 1.2;\n letter-spacing: 0.5px;\n}\n.co-intro {\n font-size: 16px;\n font-weight: 400;\n line-height: 1.75;\n color: #4D6577;\n margin-bottom: 36px;\n padding-bottom: 32px;\n border-bottom: 1px solid rgba(77,101,119,.2);\n}\n.co-wrap h2 {\n font-size: 20px;\n font-weight: 600;\n color: #323C52;\n margin: 32px 0 12px;\n letter-spacing: 0.3px;\n}\n.co-wrap p {\n font-size: 15px;\n font-weight: 400;\n color: #4D6577;\n line-height: 1.7;\n margin-bottom: 14px;\n}\n.co-list {\n list-style: none;\n padding: 0;\n margin: 12px 0 24px;\n}\n.co-list li {\n font-size: 15px;\n font-weight: 400;\n color: #4D6577;\n padding: 10px 14px 10px 38px;\n margin-bottom: 8px;\n border-radius: 6px;\n position: relative;\n background: rgba(77,101,119,.06);\n border-left: 3px solid #4D6577;\n}\n.co-list li::before {\n content: \"\u2713\";\n position: absolute;\n left: 12px;\n font-weight: 700;\n color: #4D6577;\n}\n.co-table {\n width: 100%;\n border-collapse: collapse;\n margin: 14px 0 22px;\n font-size: 14px;\n border-radius: 8px;\n overflow: hidden;\n}\n.co-table thead tr {\n background: #323C52;\n color: #fff;\n}\n.co-table thead th {\n padding: 11px 16px;\n text-align: left;\n font-weight: 600;\n font-size: 13px;\n}\n.co-table tbody tr:nth-child(even) {\n background: rgba(77,101,119,.06);\n}\n.co-table tbody tr:nth-child(odd) {\n background: #fff;\n}\n.co-table td {\n padding: 10px 16px;\n color: #4D6577;\n border-bottom: 1px solid rgba(77,101,119,.12);\n font-size: 14px;\n vertical-align: top;\n}\n.co-table td:first-child {\n font-weight: 600;\n color: #323C52;\n}\n.co-infobox {\n display: flex;\n gap: 12px;\n background: rgba(77,101,119,.06);\n border-radius: 8px;\n border-left: 4px solid #4D6577;\n padding: 14px 18px;\n margin: 18px 0 28px;\n font-size: 14px;\n font-weight: 400;\n color: #4D6577;\n line-height: 1.65;\n}\n.co-infobox .ico {\n font-size: 18px;\n flex-shrink: 0;\n}\n.co-urgence {\n background: #fff8f8;\n border-left: 5px solid #c0392b;\n border-radius: 6px;\n padding: 20px 26px;\n margin: 24px 0 32px;\n}\n.co-urgence .co-urgence-titre {\n font-size: 13px;\n font-weight: 700;\n color: #c0392b;\n letter-spacing: 1.5px;\n text-transform: uppercase;\n margin-bottom: 10px;\n}\n.co-urgence p {\n color: #5a2020;\n font-size: 14px;\n margin: 0 0 10px;\n line-height: 1.7;\n}\n.co-urgence p:last-child {\n margin-bottom: 0;\n}\n.co-disclaimer {\n font-size: 13px;\n color: #8a9aaa;\n font-style: italic;\n border-top: 1px solid rgba(77,101,119,.15);\n padding-top: 24px;\n margin-top: 40px;\n line-height: 1.6;\n}\n<\/style>\n<\/head>\n<body>\n<div class=\"co-wrap\">\n\n <span class=\"co-label\">Endocrinology & Clinical Biochemistry & Family Medicine<\/span>\n <h1>Free T4 (FT4) \u2014 Free Thyroxine<\/h1>\n\n <div class=\"co-intro\">\n Free T4 (FT4) is a measure of the fraction of thyroxine (T4)\u2014the primary thyroid hormone secreted by the thyroid gland\u2014that is not bound to transport proteins (80\u201390% of total thyroid secretion)\u2014which circulates freely in plasma and is available to be taken up by target cells and converted into active T3 by tissue deiodinases. Total circulating T4 is transported 99.97% in a form bound to plasma proteins\u2014primarily TBG (thyroxine-binding globulin \u2014 75% of total T4) + albumin (15\u201320% of total T4) + transthyretin (5\u201310% of total T4) \u2014 and accounts for only 0.03% of total T4 as the unbound free fraction (FT4). This free fraction is the only one biologically available to target cells: it diffuses freely across cell membranes and is taken up by intracellular deiodinases, which convert it into active T3 (type 2 deiodinase\u2014brain, pituitary, heart, and muscle) or inactive reverse T3 (rT3) depending on metabolic needs. T4 is a prohormone: it is biologically inactive on its own but serves as the primary substrate for peripheral conversion to T3 (the hormonally active form)\u2014similar to proinsulin, which is converted into active insulin. Measuring FT4 is more reliable than total T4 in conditions that alter transport proteins (pregnancy, estrogen, liver disease, nephrotic syndrome, medications) because FT4 reflects the biologically active fraction regardless of variations in carrier proteins. In the standard diagnostic algorithm for thyroid function, FT4 is measured as a second-line test after TSH \u2014 if TSH is abnormal \u2014 to determine the degree and type of thyroid dysfunction, and is the parameter of choice for monitoring patients on levothyroxine and for evaluating secondary (pituitary) thyroid dysfunction.\n <\/div>\n\n <h2>Normal Values and Interpretation of a Complete Thyroid Panel<\/h2>\n <ul class=\"co-list\">\n <li><strong>Reference values FT4 (adult):<\/strong> 9-25 pmol\/L (or 0.7-1.9 ng\/dL depending on the unit) + reference values vary between laboratories and enzyme immunoassay methods \u2192 always compare with the standards of the laboratory performing the analysis<\/li>\n <li><strong>Pregnancy \u2014 adjusted values:<\/strong> FT4 decreases progressively throughout pregnancy (plasma dilution + estrogen-stimulated increase in TBG \u2192 increased T4 protein binding \u2192 reduced free FT4) + pregnancy-specific TSH + FT4 targets \u2192 FT4 slightly lower than outside pregnancy is normal<\/li>\n <li><strong>Recommended diagnostic algorithm:<\/strong> TSH in 1st intention (most sensitive screening test) + if TSH abnormal \u2192 FT4 + FT3 according to context + if TSH suppressed and FT4 normal \u2192 measure FT3 (T3 hyperthyroidism possible) + if TSH high and FT4 normal \u2192 subclinical hypothyroidism + if TSH high and FT4 low \u2192 frank clinical hypothyroidism<\/li>\n <\/ul>\n\n <h2>Combined interpretation of TSH + FT4<\/h2>\n\n <table class=\"co-table\">\n <thead>\n <tr>\n <th>TSH<\/th>\n <th>FT4<\/th>\n <th>Diagnosis<\/th>\n <th>Driving<\/th>\n <\/tr>\n <\/thead>\n <tbody>\n <tr>\n <td>Elevated (> 4\u20135 mIU\/L)<\/td>\n <td>Normal<\/td>\n <td>Subclinical hypothyroidism<\/td>\n <td>Treatment discussed (pregnancy + TSH > 7\u20138 + symptoms + positive anti-TPO) + annual monitoring otherwise<\/td>\n <\/tr>\n <tr>\n <td>Elevated (> 4\u20135 mIU\/L)<\/td>\n <td>Low<\/td>\n <td>Overt clinical hypothyroidism (primary)<\/td>\n <td>Levothyroxine \u2192 TSH target 0.5\u20132.5 mIU\/L<\/td>\n <\/tr>\n <tr>\n <td>Normal (0.4\u20134 mIU\/L)<\/td>\n <td>Low<\/td>\n <td>Central hypothyroidism (pituitary or hypothalamic)<\/td>\n <td>Complete pituitary workup + pituitary MRI<\/td>\n <\/tr>\n <tr>\n <td>Undetectable (< 0.1 mIU\/L)<\/td>\n <td>Raised<\/td>\n <td>Overt hyperthyroidism<\/td>\n <td>Anti-RTSH + scintigraphy \u2192 etiological diagnosis<\/td>\n <\/tr>\n <tr>\n <td>Undetectable (< 0.1 mIU\/L)<\/td>\n <td>Normal<\/td>\n <td>Subclinical hyperthyroidism or isolated T3 hyperthyroidism<\/td>\n <td>Free T3 dose \u2192 If Free T3 is high = T3 hyperthyroidism<\/td>\n <\/tr>\n <tr>\n <td>Normal<\/td>\n <td>Raised<\/td>\n <td>Central hyperthyroidism (TSH-oma) or thyroid hormone resistance (TRH)<\/td>\n <td>pituitary MRI + bioactive TSH assay + endocrinology consultation<\/td>\n <\/tr>\n <tr>\n <td>Normal<\/td>\n <td>Normal<\/td>\n <td>Euthyroidism<\/td>\n <td>No treatment + monitoring if risk factors<\/td>\n <\/tr>\n <\/tbody>\n <\/table>\n\n <h2>Low FT4 \u2014 causes of hypothyroidism<\/h2>\n <ul class=\"co-list\">\n <li><strong>Primary hypothyroidism (high TSH + low FT4) - most frequent causes:<\/strong> Hashimoto's thyroiditis (autoimmune - most common in developed countries) + post-thyroidectomy hypothyroidism + post-irtherapy (iodine 131) + De Quervain thyroiditis (transient hypothyroid phase) + drug-induced hypothyroidism (amiodarone + lithium + interferon + anti-PD1\/PD-L1 immunotherapy) + iodine deficiency (rare in Canada - fortified foods)<\/li>\n <li><strong>Central hypothyroidism \u2014 secondary (normal or low TSH + low FT4):<\/strong> pituitary insufficiency (pituitary adenoma + pituitary surgery + cerebral radiotherapy + Sheehan syndrome = post-partum pituitary necrosis + head trauma) + hypothalamic insufficiency (craniopharyngioma + sarcoidosis + histiocytosis) \u2192 always consider associated adrenal insufficiency (hypopituitarism)<\/li>\n <li><strong>Low T4 Syndrome (Non-thyroidal Illness):<\/strong> FT4 low + FT3 low + TSH normal \u2192 prolonged fasting + severe sepsis + advanced heart failure + major surgery \u2192 metabolic adaptation \u2192 DO NOT treat \u2192 treat underlying cause<\/li>\n <\/ul>\n\n <h2>High FT4 - causes of hyperthyroidism<\/h2>\n <ul class=\"co-list\">\n <li><strong>Graves' disease (suppressed TSH + elevated FT4 + positive TRAb):<\/strong> most frequent hyperthyroidism + woman 20-50 years + autoimmunity (anti-RTSH stimulants) + diffuse goiter + exophthalmos (Graves' orbitopathy) + dermopathy (pretibial myxedema) \u2192 treatment: synthetic antithyroid drugs (carbimazole + propylthiouracil) + or iodine 131 + or thyroidectomy<\/li>\n <li><strong>Toxic adenoma + toxic multinodular goiter<\/strong> TSH suppressed + FT4 elevated + without anti-RTSH + scintigraphy: hyperfixing nodule(s) + treatment: iodine 131 + or surgery<\/li>\n <li><strong>De Quervain's thyroiditis (transient hyperthyroid phase):<\/strong> TSH suppressed + FT4 elevated + VS very high + CRP elevated + cervical pain + fever + 4-6 weeks hyperthyroidism \u2192 euthyroidism \u2192 hypothyroidism (20-30 % permanent) \u2192 beta-blockers + NSAIDs + no antithyroid drugs (no excess production)<\/li>\n <li><strong>Iodine load (amiodarone + contrast media):<\/strong> amiodarone \u2192 very rich in iodine (37 % by weight) \u2192 can cause hyperthyroidism (type 1 = exacerbated Basedow + type 2 = destructive thyroiditis) \u2192 high FT4 + low FT3 (inhibition of conversion) + variable TSH \u2192 complex thyroid profile under amiodarone<\/li>\n <li><strong>Sham thyrotoxicosis (surreptitious intake of T4 or T3):<\/strong> FT4 very high + TSH suppressed + thyroglobulin collapsed (because thyroid is inhibited without lesion) \u2192 diagnosis<\/li>\n <\/ul>\n\n <h2>Levothyroxine Monitoring \u2014 Practical Guidelines<\/h2>\n <ul class=\"co-list\">\n <li><strong>TSH target under treatment :<\/strong> Adult: TSH 0.5\u20132.5 mIU\/L (mid-normal range) + pregnancy: TSH < 2.5 mIU\/L in 1st trimester + < 3.0 mIU\/L in 2nd\u20133rd trimesters + differentiated thyroid cancer post-thyroidectomy: TSH < 0.5 mIU\/L (mild suppression) or < 0.1 mIU\/L (strong suppression) depending on recurrence risk<\/li>\n <li><strong>Main tracking parameter:<\/strong> TSH is the reference monitoring parameter under levothyroxine - it reflects the effect of exogenous T4 on the pituitary gland after 4-8 weeks of equilibrium + FT4 is generally not necessary for routine monitoring if TSH is within target + measure FT4 if TSH is discordant or if there is a suspected absorption problem<\/li>\n <li><strong>Control delay<\/strong> TSH control 4-8 weeks after each levothyroxine dose change (time needed to reach steady state - T4 half-life = 7 days) + once stable: TSH every 6-12 months<\/li>\n <li><strong>Taking levothyroxine:<\/strong> fasting in the morning 30-60 min before meal + or at bedtime (\u2265 3 h after last meal) + absorption reduced by coffee + calcium + iron + antacids + cholestyramine \u2192 take at a distance from these substances (\u2265 4 h)<\/li>\n <li><strong>High FT4 on levothyroxine:<\/strong> overdose \u2192 TSH suppressed + FT4 high + risk of atrial fibrillation + osteoporosis (in postmenopausal women) + reduce dose + TSH check in 4-8 weeks<\/li>\n <\/ul>\n\n <div class=\"co-infobox\">\n <span class=\"ico\">\u2119\ufe0f<\/span>\n <span>In central hypothyroidism (normal or low TSH + low FT4), TSH alone is insufficient as a monitoring test \u2014 FT4 should guide levothyroxine dose adjustment. Target an FT4 in the upper half of the normal range. This scenario is important to recognize because it can be mistaken for euthyroidism if only TSH is measured. Always measure FT4 if pituitary insufficiency is known or suspected.<\/span>\n <\/div>\n\n <div class=\"co-urgence\">\n <div class=\"co-urgence-titre\">Medical consultation recommended<\/div>\n <p>Consult a doctor if a thyroid test shows a high TSH with a low FT4 (clinical hypothyroidism requiring levothyroxine) or a suppressed TSH with a high FT4 (hyperthyroidism requiring etiological assessment and treatment) or a discordant picture (normal TSH + low FT4 \u2192 suspicion of central hypothyroidism). For the prescription and interpretation of a complete thyroid assessment (TSH + FT4 + FT3 depending on the context) and the initiation of levothyroxine, Clinique Omicron offers medical consultations at its service points in Quebec and via telemedicine. To make an appointment, visit <a href=\"https:\/\/cliniqueomicron.ca\" style=\"color:#c0392b;font-weight:600;text-decoration:none;\">cliniqueomicron.ca<\/a>.<\/p>\n <\/div>\n\n <h2>Consult at Clinique Omicron<\/h2>\n <p>Clinique Omicron's physicians and nurse practitioners (NPs) prescribe and interpret FT4 in the appropriate clinical context (hypothyroidism screening, treatment monitoring, pregnancy assessment, hyperthyroidism evaluation, central hypothyroidism), initiate and adjust levothyroxine according to TSH goals based on the clinical context, recognize discordant presentations (normal TSH + low FT4 = central hypothyroidism) requiring pituitary investigation, and refer to endocrinology for complex cases. Consultations are available at multiple service points across Quebec and via telemedicine. To book an appointment, visit <a href=\"https:\/\/cliniqueomicron.ca\" style=\"color:#4D6577;font-weight:600;text-decoration:none;\">cliniqueomicron.ca<\/a>.<\/p>\n\n <p class=\"co-disclaimer\">The content of this page is for informational purposes only and does not substitute for medical or endocrinological advice. FT4 should always be interpreted in conjunction with TSH and within the patient's clinical context. In central hypothyroidism, TSH may be normal despite low FT4 \u2014 FT4 is the primary monitoring parameter in this setting. Reference ranges vary by laboratory.<\/p>\n\n<\/div>\n<\/body>\n<\/html>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<\/div>","protected":false},"excerpt":{"rendered":"<p>T4 libre (FT4) : thyroxine libre, valeurs normales et interpr\u00e9tation thyro\u00efdienne | Clinique Omicron Endocrinologie & Biochimie clinique & M\u00e9decine de famille T4 libre (FT4) \u2014 Thyroxine libre La T4 libre (FT4 \u2014 Free Thyroxine ou Thyroxine libre) est la mesure de la fraction non li\u00e9e aux prot\u00e9ines de transport de la thyroxine (T4) \u2014… <a href=\"https:\/\/cliniqueomicron.ca\/en\/t4-libre\/\" rel=\"bookmark\">Read More \"<span class=\"screen-reader-text\">Free T4 (FT4): free thyroxine, normal values and thyroid interpretation | Clinique Omicron<\/span><\/a><\/p>","protected":false},"author":1,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"om_disable_all_campaigns":false,"_monsterinsights_skip_tracking":false,"_monsterinsights_sitenote_active":false,"_monsterinsights_sitenote_note":"","_monsterinsights_sitenote_category":0,"neve_meta_sidebar":"","neve_meta_container":"","neve_meta_enable_content_width":"off","neve_meta_content_width":100,"neve_meta_title_alignment":"","neve_meta_author_avatar":"","neve_post_elements_order":"","neve_meta_disable_header":"","neve_meta_disable_footer":"","neve_meta_disable_title":"","_themeisle_gutenberg_block_has_review":false,"_metasync_otto_title":"T4 libre (thyroxine) | Brossard | Clinique Omicron","_metasync_otto_description":"T4 libre (thyroxine) T4 libre (thyroxine) est un test de laboratoire qui permet d'\u00e9valuer certains aspects de votre \u00e9tat de sant\u00e9. Ce test est disponible \u00e0 l...","_metasync_otto_keywords":"","_metasync_otto_og_title":"","_metasync_otto_og_description":"T4 libre (thyroxine) T4 libre (thyroxine) est un test de laboratoire qui permet d'\u00e9valuer certains aspects de votre \u00e9tat de sant\u00e9. Ce test est disponible \u00e0 l...","_metasync_otto_twitter_title":"","_metasync_otto_twitter_description":"T4 libre (thyroxine) T4 libre (thyroxine) est un test de laboratoire qui permet d'\u00e9valuer certains aspects de votre \u00e9tat de sant\u00e9. Ce test est disponible \u00e0 l...","rank_math_title":"","rank_math_description":"","_yoast_wpseo_title":"","_yoast_wpseo_metadesc":"","_aioseo_title":"T4 libre (FT4) : thyroxine libre, valeurs normales et interpr\u00e9tation thyro\u00efdienne | Clinique Omicron","_aioseo_description":"La FT4 est la fraction active de la thyroxine non li\u00e9e aux prot\u00e9ines. TSH \u00e9lev\u00e9e FT4 basse hypothyro\u00efdie clinique l\u00e9vothyroxine, TSH normale FT4 basse hypothyro\u00efdie centrale hypophysaire, TSH supprim\u00e9e FT4 \u00e9lev\u00e9e hyperthyro\u00efdie au Qu\u00e9bec.","_metasync_seo_title":"","_metasync_seo_desc":"","_metasync_breadcrumb_title":"","_metasync_primary_category":0,"_metasync_primary_product_cat":0,"_metasync_otto_disabled":"","_metasync_hreflang":"","_metasync_plugin_sync_ts":"{\"aioseo\":\"2026-05-10T07:30:10+00:00\"}","_metasync_robots_advanced":"","footnotes":""},"class_list":["post-24991","page","type-page","status-publish","hentry"],"aioseo_notices":[],"_links":{"self":[{"href":"https:\/\/cliniqueomicron.ca\/en\/wp-json\/wp\/v2\/pages\/24991","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/cliniqueomicron.ca\/en\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/cliniqueomicron.ca\/en\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/cliniqueomicron.ca\/en\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/cliniqueomicron.ca\/en\/wp-json\/wp\/v2\/comments?post=24991"}],"version-history":[{"count":4,"href":"https:\/\/cliniqueomicron.ca\/en\/wp-json\/wp\/v2\/pages\/24991\/revisions"}],"predecessor-version":[{"id":31916,"href":"https:\/\/cliniqueomicron.ca\/en\/wp-json\/wp\/v2\/pages\/24991\/revisions\/31916"}],"wp:attachment":[{"href":"https:\/\/cliniqueomicron.ca\/en\/wp-json\/wp\/v2\/media?parent=24991"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}

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\n\t\t\t\t\t\n\n\n\n\nFree T4 (FT4): free thyroxine, normal values and thyroid interpretation | Clinique Omicron<\/title>\n<meta name=\"description\" content=\"Free T4 (FT4) is the active fraction of thyroxine not bound to proteins. Low FT4 hypothyroidism, high FT4 hyperthyroidism, TSH FT4 FT3 combined interpretation and levothyroxine monitoring in Quebec.\">\n<meta name=\"keywords\" content=\"T4 libre, FT4, thyroxine libre, T4 libre interpr\u00e9tation, FT4 hypothyro\u00efdie, FT4 hyperthyro\u00efdie, TSH FT4 bilan thyro\u00efdien, FT4 valeurs normales, T4 libre suivi l\u00e9vothyroxine, T4 libre Qu\u00e9bec\">\n<link rel=\"preconnect\" href=\"https:\/\/fonts.googleapis.com\">\n<link href=\"https:\/\/fonts.googleapis.com\/css2?family=Cinzel:wght@600&family=Poppins:wght@400;500;600;700&display=swap\" rel=\"stylesheet\">\n<style>\n@import url('https:\/\/fonts.googleapis.com\/css2?family=Cinzel:wght@600&family=Poppins:wght@400;500;600;700&display=swap');\n\n.co-wrap * {\n font-family: 'Poppins', sans-serif;\n box-sizing: border-box;\n}\n.co-wrap {\n max-width: 1100px;\n margin: 0 auto;\n padding: 30px 0 60px;\n margin-top: 10px;\n}\n.co-label {\n font-family: 'Cinzel', serif;\n font-size: 14px;\n font-weight: bold;\n letter-spacing: 1px;\n text-transform: uppercase;\n color: #4D6577;\n margin-bottom: 14px;\n display: block;\n}\n.co-wrap h1 {\n font-size: 32px;\n font-weight: 500;\n color: #323C52;\n margin: 0 0 22px;\n line-height: 1.2;\n letter-spacing: 0.5px;\n}\n.co-intro {\n font-size: 16px;\n font-weight: 400;\n line-height: 1.75;\n color: #4D6577;\n margin-bottom: 36px;\n padding-bottom: 32px;\n border-bottom: 1px solid rgba(77,101,119,.2);\n}\n.co-wrap h2 {\n font-size: 20px;\n font-weight: 600;\n color: #323C52;\n margin: 32px 0 12px;\n letter-spacing: 0.3px;\n}\n.co-wrap p {\n font-size: 15px;\n font-weight: 400;\n color: #4D6577;\n line-height: 1.7;\n margin-bottom: 14px;\n}\n.co-list {\n list-style: none;\n padding: 0;\n margin: 12px 0 24px;\n}\n.co-list li {\n font-size: 15px;\n font-weight: 400;\n color: #4D6577;\n padding: 10px 14px 10px 38px;\n margin-bottom: 8px;\n border-radius: 6px;\n position: relative;\n background: rgba(77,101,119,.06);\n border-left: 3px solid #4D6577;\n}\n.co-list li::before {\n content: \"\u2713\";\n position: absolute;\n left: 12px;\n font-weight: 700;\n color: #4D6577;\n}\n.co-table {\n width: 100%;\n border-collapse: collapse;\n margin: 14px 0 22px;\n font-size: 14px;\n border-radius: 8px;\n overflow: hidden;\n}\n.co-table thead tr {\n background: #323C52;\n color: #fff;\n}\n.co-table thead th {\n padding: 11px 16px;\n text-align: left;\n font-weight: 600;\n font-size: 13px;\n}\n.co-table tbody tr:nth-child(even) {\n background: rgba(77,101,119,.06);\n}\n.co-table tbody tr:nth-child(odd) {\n background: #fff;\n}\n.co-table td {\n padding: 10px 16px;\n color: #4D6577;\n border-bottom: 1px solid rgba(77,101,119,.12);\n font-size: 14px;\n vertical-align: top;\n}\n.co-table td:first-child {\n font-weight: 600;\n color: #323C52;\n}\n.co-infobox {\n display: flex;\n gap: 12px;\n background: rgba(77,101,119,.06);\n border-radius: 8px;\n border-left: 4px solid #4D6577;\n padding: 14px 18px;\n margin: 18px 0 28px;\n font-size: 14px;\n font-weight: 400;\n color: #4D6577;\n line-height: 1.65;\n}\n.co-infobox .ico {\n font-size: 18px;\n flex-shrink: 0;\n}\n.co-urgence {\n background: #fff8f8;\n border-left: 5px solid #c0392b;\n border-radius: 6px;\n padding: 20px 26px;\n margin: 24px 0 32px;\n}\n.co-urgence .co-urgence-titre {\n font-size: 13px;\n font-weight: 700;\n color: #c0392b;\n letter-spacing: 1.5px;\n text-transform: uppercase;\n margin-bottom: 10px;\n}\n.co-urgence p {\n color: #5a2020;\n font-size: 14px;\n margin: 0 0 10px;\n line-height: 1.7;\n}\n.co-urgence p:last-child {\n margin-bottom: 0;\n}\n.co-disclaimer {\n font-size: 13px;\n color: #8a9aaa;\n font-style: italic;\n border-top: 1px solid rgba(77,101,119,.15);\n padding-top: 24px;\n margin-top: 40px;\n line-height: 1.6;\n}\n<\/style>\n<\/head>\n<body>\n<div class=\"co-wrap\">\n\n <span class=\"co-label\">Endocrinology & Clinical Biochemistry & Family Medicine<\/span>\n <h1>Free T4 (FT4) \u2014 Free Thyroxine<\/h1>\n\n <div class=\"co-intro\">\n Free T4 (FT4) is a measure of the fraction of thyroxine (T4)\u2014the primary thyroid hormone secreted by the thyroid gland\u2014that is not bound to transport proteins (80\u201390% of total thyroid secretion)\u2014which circulates freely in plasma and is available to be taken up by target cells and converted into active T3 by tissue deiodinases. Total circulating T4 is transported 99.97% in a form bound to plasma proteins\u2014primarily TBG (thyroxine-binding globulin \u2014 75% of total T4) + albumin (15\u201320% of total T4) + transthyretin (5\u201310% of total T4) \u2014 and accounts for only 0.03% of total T4 as the unbound free fraction (FT4). This free fraction is the only one biologically available to target cells: it diffuses freely across cell membranes and is taken up by intracellular deiodinases, which convert it into active T3 (type 2 deiodinase\u2014brain, pituitary, heart, and muscle) or inactive reverse T3 (rT3) depending on metabolic needs. T4 is a prohormone: it is biologically inactive on its own but serves as the primary substrate for peripheral conversion to T3 (the hormonally active form)\u2014similar to proinsulin, which is converted into active insulin. Measuring FT4 is more reliable than total T4 in conditions that alter transport proteins (pregnancy, estrogen, liver disease, nephrotic syndrome, medications) because FT4 reflects the biologically active fraction regardless of variations in carrier proteins. In the standard diagnostic algorithm for thyroid function, FT4 is measured as a second-line test after TSH \u2014 if TSH is abnormal \u2014 to determine the degree and type of thyroid dysfunction, and is the parameter of choice for monitoring patients on levothyroxine and for evaluating secondary (pituitary) thyroid dysfunction.\n <\/div>\n\n <h2>Normal Values and Interpretation of a Complete Thyroid Panel<\/h2>\n <ul class=\"co-list\">\n <li><strong>Reference values FT4 (adult):<\/strong> 9-25 pmol\/L (or 0.7-1.9 ng\/dL depending on the unit) + reference values vary between laboratories and enzyme immunoassay methods \u2192 always compare with the standards of the laboratory performing the analysis<\/li>\n <li><strong>Pregnancy \u2014 adjusted values:<\/strong> FT4 decreases progressively throughout pregnancy (plasma dilution + estrogen-stimulated increase in TBG \u2192 increased T4 protein binding \u2192 reduced free FT4) + pregnancy-specific TSH + FT4 targets \u2192 FT4 slightly lower than outside pregnancy is normal<\/li>\n <li><strong>Recommended diagnostic algorithm:<\/strong> TSH in 1st intention (most sensitive screening test) + if TSH abnormal \u2192 FT4 + FT3 according to context + if TSH suppressed and FT4 normal \u2192 measure FT3 (T3 hyperthyroidism possible) + if TSH high and FT4 normal \u2192 subclinical hypothyroidism + if TSH high and FT4 low \u2192 frank clinical hypothyroidism<\/li>\n <\/ul>\n\n <h2>Combined interpretation of TSH + FT4<\/h2>\n\n <table class=\"co-table\">\n <thead>\n <tr>\n <th>TSH<\/th>\n <th>FT4<\/th>\n <th>Diagnosis<\/th>\n <th>Driving<\/th>\n <\/tr>\n <\/thead>\n <tbody>\n <tr>\n <td>Elevated (> 4\u20135 mIU\/L)<\/td>\n <td>Normal<\/td>\n <td>Subclinical hypothyroidism<\/td>\n <td>Treatment discussed (pregnancy + TSH > 7\u20138 + symptoms + positive anti-TPO) + annual monitoring otherwise<\/td>\n <\/tr>\n <tr>\n <td>Elevated (> 4\u20135 mIU\/L)<\/td>\n <td>Low<\/td>\n <td>Overt clinical hypothyroidism (primary)<\/td>\n <td>Levothyroxine \u2192 TSH target 0.5\u20132.5 mIU\/L<\/td>\n <\/tr>\n <tr>\n <td>Normal (0.4\u20134 mIU\/L)<\/td>\n <td>Low<\/td>\n <td>Central hypothyroidism (pituitary or hypothalamic)<\/td>\n <td>Complete pituitary workup + pituitary MRI<\/td>\n <\/tr>\n <tr>\n <td>Undetectable (< 0.1 mIU\/L)<\/td>\n <td>Raised<\/td>\n <td>Overt hyperthyroidism<\/td>\n <td>Anti-RTSH + scintigraphy \u2192 etiological diagnosis<\/td>\n <\/tr>\n <tr>\n <td>Undetectable (< 0.1 mIU\/L)<\/td>\n <td>Normal<\/td>\n <td>Subclinical hyperthyroidism or isolated T3 hyperthyroidism<\/td>\n <td>Free T3 dose \u2192 If Free T3 is high = T3 hyperthyroidism<\/td>\n <\/tr>\n <tr>\n <td>Normal<\/td>\n <td>Raised<\/td>\n <td>Central hyperthyroidism (TSH-oma) or thyroid hormone resistance (TRH)<\/td>\n <td>pituitary MRI + bioactive TSH assay + endocrinology consultation<\/td>\n <\/tr>\n <tr>\n <td>Normal<\/td>\n <td>Normal<\/td>\n <td>Euthyroidism<\/td>\n <td>No treatment + monitoring if risk factors<\/td>\n <\/tr>\n <\/tbody>\n <\/table>\n\n <h2>Low FT4 \u2014 causes of hypothyroidism<\/h2>\n <ul class=\"co-list\">\n <li><strong>Primary hypothyroidism (high TSH + low FT4) - most frequent causes:<\/strong> Hashimoto's thyroiditis (autoimmune - most common in developed countries) + post-thyroidectomy hypothyroidism + post-irtherapy (iodine 131) + De Quervain thyroiditis (transient hypothyroid phase) + drug-induced hypothyroidism (amiodarone + lithium + interferon + anti-PD1\/PD-L1 immunotherapy) + iodine deficiency (rare in Canada - fortified foods)<\/li>\n <li><strong>Central hypothyroidism \u2014 secondary (normal or low TSH + low FT4):<\/strong> pituitary insufficiency (pituitary adenoma + pituitary surgery + cerebral radiotherapy + Sheehan syndrome = post-partum pituitary necrosis + head trauma) + hypothalamic insufficiency (craniopharyngioma + sarcoidosis + histiocytosis) \u2192 always consider associated adrenal insufficiency (hypopituitarism)<\/li>\n <li><strong>Low T4 Syndrome (Non-thyroidal Illness):<\/strong> FT4 low + FT3 low + TSH normal \u2192 prolonged fasting + severe sepsis + advanced heart failure + major surgery \u2192 metabolic adaptation \u2192 DO NOT treat \u2192 treat underlying cause<\/li>\n <\/ul>\n\n <h2>High FT4 - causes of hyperthyroidism<\/h2>\n <ul class=\"co-list\">\n <li><strong>Graves' disease (suppressed TSH + elevated FT4 + positive TRAb):<\/strong> most frequent hyperthyroidism + woman 20-50 years + autoimmunity (anti-RTSH stimulants) + diffuse goiter + exophthalmos (Graves' orbitopathy) + dermopathy (pretibial myxedema) \u2192 treatment: synthetic antithyroid drugs (carbimazole + propylthiouracil) + or iodine 131 + or thyroidectomy<\/li>\n <li><strong>Toxic adenoma + toxic multinodular goiter<\/strong> TSH suppressed + FT4 elevated + without anti-RTSH + scintigraphy: hyperfixing nodule(s) + treatment: iodine 131 + or surgery<\/li>\n <li><strong>De Quervain's thyroiditis (transient hyperthyroid phase):<\/strong> TSH suppressed + FT4 elevated + VS very high + CRP elevated + cervical pain + fever + 4-6 weeks hyperthyroidism \u2192 euthyroidism \u2192 hypothyroidism (20-30 % permanent) \u2192 beta-blockers + NSAIDs + no antithyroid drugs (no excess production)<\/li>\n <li><strong>Iodine load (amiodarone + contrast media):<\/strong> amiodarone \u2192 very rich in iodine (37 % by weight) \u2192 can cause hyperthyroidism (type 1 = exacerbated Basedow + type 2 = destructive thyroiditis) \u2192 high FT4 + low FT3 (inhibition of conversion) + variable TSH \u2192 complex thyroid profile under amiodarone<\/li>\n <li><strong>Sham thyrotoxicosis (surreptitious intake of T4 or T3):<\/strong> FT4 very high + TSH suppressed + thyroglobulin collapsed (because thyroid is inhibited without lesion) \u2192 diagnosis<\/li>\n <\/ul>\n\n <h2>Levothyroxine Monitoring \u2014 Practical Guidelines<\/h2>\n <ul class=\"co-list\">\n <li><strong>TSH target under treatment :<\/strong> Adult: TSH 0.5\u20132.5 mIU\/L (mid-normal range) + pregnancy: TSH < 2.5 mIU\/L in 1st trimester + < 3.0 mIU\/L in 2nd\u20133rd trimesters + differentiated thyroid cancer post-thyroidectomy: TSH < 0.5 mIU\/L (mild suppression) or < 0.1 mIU\/L (strong suppression) depending on recurrence risk<\/li>\n <li><strong>Main tracking parameter:<\/strong> TSH is the reference monitoring parameter under levothyroxine - it reflects the effect of exogenous T4 on the pituitary gland after 4-8 weeks of equilibrium + FT4 is generally not necessary for routine monitoring if TSH is within target + measure FT4 if TSH is discordant or if there is a suspected absorption problem<\/li>\n <li><strong>Control delay<\/strong> TSH control 4-8 weeks after each levothyroxine dose change (time needed to reach steady state - T4 half-life = 7 days) + once stable: TSH every 6-12 months<\/li>\n <li><strong>Taking levothyroxine:<\/strong> fasting in the morning 30-60 min before meal + or at bedtime (\u2265 3 h after last meal) + absorption reduced by coffee + calcium + iron + antacids + cholestyramine \u2192 take at a distance from these substances (\u2265 4 h)<\/li>\n <li><strong>High FT4 on levothyroxine:<\/strong> overdose \u2192 TSH suppressed + FT4 high + risk of atrial fibrillation + osteoporosis (in postmenopausal women) + reduce dose + TSH check in 4-8 weeks<\/li>\n <\/ul>\n\n <div class=\"co-infobox\">\n <span class=\"ico\">\u2119\ufe0f<\/span>\n <span>In central hypothyroidism (normal or low TSH + low FT4), TSH alone is insufficient as a monitoring test \u2014 FT4 should guide levothyroxine dose adjustment. Target an FT4 in the upper half of the normal range. This scenario is important to recognize because it can be mistaken for euthyroidism if only TSH is measured. Always measure FT4 if pituitary insufficiency is known or suspected.<\/span>\n <\/div>\n\n <div class=\"co-urgence\">\n <div class=\"co-urgence-titre\">Medical consultation recommended<\/div>\n <p>Consult a doctor if a thyroid test shows a high TSH with a low FT4 (clinical hypothyroidism requiring levothyroxine) or a suppressed TSH with a high FT4 (hyperthyroidism requiring etiological assessment and treatment) or a discordant picture (normal TSH + low FT4 \u2192 suspicion of central hypothyroidism). For the prescription and interpretation of a complete thyroid assessment (TSH + FT4 + FT3 depending on the context) and the initiation of levothyroxine, Clinique Omicron offers medical consultations at its service points in Quebec and via telemedicine. To make an appointment, visit <a href=\"https:\/\/cliniqueomicron.ca\" style=\"color:#c0392b;font-weight:600;text-decoration:none;\">cliniqueomicron.ca<\/a>.<\/p>\n <\/div>\n\n <h2>Consult at Clinique Omicron<\/h2>\n <p>Clinique Omicron's physicians and nurse practitioners (NPs) prescribe and interpret FT4 in the appropriate clinical context (hypothyroidism screening, treatment monitoring, pregnancy assessment, hyperthyroidism evaluation, central hypothyroidism), initiate and adjust levothyroxine according to TSH goals based on the clinical context, recognize discordant presentations (normal TSH + low FT4 = central hypothyroidism) requiring pituitary investigation, and refer to endocrinology for complex cases. Consultations are available at multiple service points across Quebec and via telemedicine. To book an appointment, visit <a href=\"https:\/\/cliniqueomicron.ca\" style=\"color:#4D6577;font-weight:600;text-decoration:none;\">cliniqueomicron.ca<\/a>.<\/p>\n\n <p class=\"co-disclaimer\">The content of this page is for informational purposes only and does not substitute for medical or endocrinological advice. FT4 should always be interpreted in conjunction with TSH and within the patient's clinical context. In central hypothyroidism, TSH may be normal despite low FT4 \u2014 FT4 is the primary monitoring parameter in this setting. Reference ranges vary by laboratory.<\/p>\n\n<\/div>\n<\/body>\n<\/html>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<\/div>","protected":false},"excerpt":{"rendered":"<p>T4 libre (FT4) : thyroxine libre, valeurs normales et interpr\u00e9tation thyro\u00efdienne | Clinique Omicron Endocrinologie & Biochimie clinique & M\u00e9decine de famille T4 libre (FT4) \u2014 Thyroxine libre La T4 libre (FT4 \u2014 Free Thyroxine ou Thyroxine libre) est la mesure de la fraction non li\u00e9e aux prot\u00e9ines de transport de la thyroxine (T4) \u2014… <a href=\"https:\/\/cliniqueomicron.ca\/en\/t4-libre\/\" rel=\"bookmark\">Read More \"<span class=\"screen-reader-text\">Free T4 (FT4): free thyroxine, normal values and thyroid interpretation | Clinique Omicron<\/span><\/a><\/p>","protected":false},"author":1,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"om_disable_all_campaigns":false,"_monsterinsights_skip_tracking":false,"_monsterinsights_sitenote_active":false,"_monsterinsights_sitenote_note":"","_monsterinsights_sitenote_category":0,"neve_meta_sidebar":"","neve_meta_container":"","neve_meta_enable_content_width":"off","neve_meta_content_width":100,"neve_meta_title_alignment":"","neve_meta_author_avatar":"","neve_post_elements_order":"","neve_meta_disable_header":"","neve_meta_disable_footer":"","neve_meta_disable_title":"","_themeisle_gutenberg_block_has_review":false,"_metasync_otto_title":"T4 libre (thyroxine) | Brossard | Clinique Omicron","_metasync_otto_description":"T4 libre (thyroxine) T4 libre (thyroxine) est un test de laboratoire qui permet d'\u00e9valuer certains aspects de votre \u00e9tat de sant\u00e9. Ce test est disponible \u00e0 l...","_metasync_otto_keywords":"","_metasync_otto_og_title":"","_metasync_otto_og_description":"T4 libre (thyroxine) T4 libre (thyroxine) est un test de laboratoire qui permet d'\u00e9valuer certains aspects de votre \u00e9tat de sant\u00e9. Ce test est disponible \u00e0 l...","_metasync_otto_twitter_title":"","_metasync_otto_twitter_description":"T4 libre (thyroxine) T4 libre (thyroxine) est un test de laboratoire qui permet d'\u00e9valuer certains aspects de votre \u00e9tat de sant\u00e9. Ce test est disponible \u00e0 l...","rank_math_title":"","rank_math_description":"","_yoast_wpseo_title":"","_yoast_wpseo_metadesc":"","_aioseo_title":"T4 libre (FT4) : thyroxine libre, valeurs normales et interpr\u00e9tation thyro\u00efdienne | Clinique Omicron","_aioseo_description":"La FT4 est la fraction active de la thyroxine non li\u00e9e aux prot\u00e9ines. TSH \u00e9lev\u00e9e FT4 basse hypothyro\u00efdie clinique l\u00e9vothyroxine, TSH normale FT4 basse hypothyro\u00efdie centrale hypophysaire, TSH supprim\u00e9e FT4 \u00e9lev\u00e9e hyperthyro\u00efdie au Qu\u00e9bec.","_metasync_seo_title":"","_metasync_seo_desc":"","_metasync_breadcrumb_title":"","_metasync_primary_category":0,"_metasync_primary_product_cat":0,"_metasync_otto_disabled":"","_metasync_hreflang":"","_metasync_plugin_sync_ts":"{\"aioseo\":\"2026-05-10T07:30:10+00:00\"}","_metasync_robots_advanced":"","footnotes":""},"class_list":["post-24991","page","type-page","status-publish","hentry"],"aioseo_notices":[],"_links":{"self":[{"href":"https:\/\/cliniqueomicron.ca\/en\/wp-json\/wp\/v2\/pages\/24991","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/cliniqueomicron.ca\/en\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/cliniqueomicron.ca\/en\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/cliniqueomicron.ca\/en\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/cliniqueomicron.ca\/en\/wp-json\/wp\/v2\/comments?post=24991"}],"version-history":[{"count":4,"href":"https:\/\/cliniqueomicron.ca\/en\/wp-json\/wp\/v2\/pages\/24991\/revisions"}],"predecessor-version":[{"id":31916,"href":"https:\/\/cliniqueomicron.ca\/en\/wp-json\/wp\/v2\/pages\/24991\/revisions\/31916"}],"wp:attachment":[{"href":"https:\/\/cliniqueomicron.ca\/en\/wp-json\/wp\/v2\/media?parent=24991"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}