Lab result explainedThyroidLast updated November 8, 2026

Free T4 blood test: what high and low free thyroxine levels mean

Free thyroxine (free T4) (FT4) · Thyroid panel (TSH with free T4, often free T3) · reported in ng/dL (SI units: pmol/L; 1 ng/dL = 12.87 pmol/L)

Free thyroxine (free T4 or FT4) is the portion of the thyroid hormone thyroxine that is not bound to carrier proteins and is therefore available to enter your cells. It is the hormone your thyroid gland actually makes most of, and it is converted in the liver, kidneys and other tissues into the more active T3. A normal free T4 on most US lab reports is roughly 0.8 to 1.8 ng/dL, though the exact range depends on the assay. Most people have it checked alongside TSH, and the two numbers are always read together.

A low free T4 usually means the thyroid is under-producing hormone (hypothyroidism), most often from Hashimoto's thyroiditis, thyroid surgery or radioactive iodine treatment, or from too low a dose of levothyroxine. A high free T4 usually means the thyroid is over-producing (hyperthyroidism), most often from Graves' disease, an overactive nodule or an inflamed thyroid, or from too high a dose of thyroid medication.

Because TSH from the pituitary responds to free T4 with a logarithmic sensitivity, TSH moves first and furthest. Free T4 often stays inside the normal range while TSH is already abnormal; that pattern is called subclinical thyroid disease. Free T4 earns its place when TSH is abnormal and you need to know how far the thyroid has drifted, when pituitary disease is possible (in which case TSH cannot be trusted), and when adjusting treatment for hyperthyroidism.

FT4 at a glance

What it is: the unbound, biologically available form of thyroxine (T4), the main hormone your thyroid gland produces.
Normal: about 0.8–1.8 ng/dL on most US assays (10–23 pmol/L in SI units); ranges vary by lab and are narrower in pregnancy.
Low free T4 with high TSH: overt hypothyroidism (underactive thyroid). Normal free T4 with high TSH: subclinical hypothyroidism.
High free T4 with low TSH: overt hyperthyroidism. Normal free T4 with low TSH: subclinical hyperthyroidism (check free T3 too).
Low free T4 with a low or normal TSH points to a pituitary or hypothalamic problem (central hypothyroidism), not the thyroid itself.
Biotin supplements can make free T4 read falsely high and TSH falsely low; stop biotin for at least 2 to 3 days before the test.

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What FT4 measures

Your thyroid releases about 80 to 100 micrograms of thyroxine (T4) a day, plus a small amount of triiodothyronine (T3). More than 99.9 percent of the T4 in blood is bound to thyroxine-binding globulin (TBG), transthyretin and albumin and is inactive while bound. Only the free fraction, about 0.03 percent, can enter cells, where enzymes called deiodinases strip an iodine atom to make T3, the form that binds the thyroid hormone receptor. Free T4 is the clinically useful measurement because total T4 rises and falls with the binding proteins, which change with estrogen, pregnancy, liver disease and many drugs, without any change in thyroid function.

Most laboratories measure free T4 with an automated immunoassay that estimates the free fraction indirectly. These assays are good in healthy people but can be distorted by very high or low binding proteins, by heparin (which releases fatty acids that displace T4), by severe non-thyroidal illness, and by biotin, which interferes with the streptavidin-biotin chemistry many assays use. Equilibrium dialysis followed by mass spectrometry is the reference method and is used when the immunoassay result does not fit the clinical picture.

Free T4 has a half-life of about 7 days, so it changes slowly. After a dose change of levothyroxine, free T4 settles within 2 to 3 weeks but TSH takes 6 to 8 weeks to reach its new equilibrium, which is why repeat testing after a dose change is usually scheduled at 6 to 8 weeks. Taking a levothyroxine tablet within a few hours of the blood draw raises free T4 by 10 to 20 percent for several hours, so most clinicians ask you to take the dose after the test that day.

Normal range for FT4

Reference ranges vary between labs and by age, sex and pregnancy. Use the range printed on your own report.

Group	Typical range (ng/dL (SI units: pmol/L; 1 ng/dL = 12.87 pmol/L))	Notes
Adults, typical immunoassay reference range	0.8–1.8 ng/dL (10–23 pmol/L)	Common assay ranges include 0.82–1.77, 0.9–1.7 and 0.7–1.5 ng/dL. Always use the range printed on your own report.
Overt hypothyroidism	Below the lab's lower limit (typically under 0.8 ng/dL) with TSH above 10 mIU/L	Values under 0.4 ng/dL with a TSH over 50 indicate severe hypothyroidism.
Subclinical hypothyroidism	Normal free T4 with TSH 4.5–10 mIU/L	Affects 4 to 10 percent of adults; many revert to normal on repeat testing.
Overt hyperthyroidism	Above the lab's upper limit (typically over 1.8 ng/dL) with TSH below 0.1 mIU/L	Graves' disease often produces 2.5–6 ng/dL; values above 5–7 ng/dL with fever or confusion raise concern for thyroid storm.
Subclinical hyperthyroidism	Normal free T4 (and free T3) with TSH below 0.4 mIU/L	Repeat in 2 to 3 months; persistent TSH under 0.1 carries a higher risk of atrial fibrillation and bone loss.
Central (pituitary) hypothyroidism	Low free T4 with TSH low, normal or only mildly raised	TSH is unreliable here; free T4 is the diagnostic test.
Pregnancy, first trimester	Upper end of normal or slightly above	hCG stimulates the thyroid; TSH falls and free T4 rises transiently.
Pregnancy, second and third trimester	Lower than non-pregnant, by about 10–30 percent on immunoassays	Binding proteins rise and the assay reads low; trimester-specific ranges should be used.
Newborns	Higher than adults in the first weeks of life	Newborn screening uses TSH and T4 on heel-prick blood with age-specific cut-offs.

High free T4

A high free T4 with a suppressed TSH (below 0.1 mIU/L) means overt hyperthyroidism: your thyroid is releasing more hormone than your body needs, and the pituitary has switched off TSH in response. In the United States the most common cause is Graves' disease, an autoimmune condition in which antibodies stimulate the TSH receptor. In older adults, toxic multinodular goiter and a single autonomous (toxic) nodule become more common. A tender thyroid after a viral illness, or a painless post-partum thyroiditis, releases stored hormone and gives a high free T4 that usually settles within weeks to months.

In someone taking levothyroxine, a high free T4 with low TSH most often means the dose is too high, or that the tablet was taken shortly before the blood draw. Over-replacement is common: in large US cohorts about 15 to 20 percent of people on levothyroxine have a suppressed TSH, and sustained over-treatment raises the risk of atrial fibrillation and osteoporosis.

A high free T4 with a normal or raised TSH is unusual and should not be accepted at face value. Biotin interference, antibodies that interfere with the assay, familial dysalbuminemic hyperthyroxinemia, amiodarone treatment, and the rare TSH-secreting pituitary tumor or thyroid hormone resistance all produce this pattern. The first step is to stop biotin, repeat the test, and if the pattern persists, measure free T4 by equilibrium dialysis and refer to endocrinology.

Common causes

Graves' disease
Thyroid-stimulating antibodies drive the whole gland to over-produce hormone. Free T4 is commonly 2 to 6 ng/dL with TSH under 0.01 mIU/L; TSH-receptor antibodies (TRAb) confirm the diagnosis. Most common cause in people under 50, and 5 to 10 times more common in women.
Toxic multinodular goiter or a toxic adenoma
One or more nodules make hormone independently of TSH. More common after 60 and in regions of past iodine deficiency; free T4 is usually only mildly raised and T3 may be disproportionately high. A radioactive iodine uptake scan shows hot nodules.
Thyroiditis (subacute, silent, post-partum, drug-induced)
Inflammation leaks preformed hormone into the blood for 4 to 8 weeks, often followed by a hypothyroid phase before recovery. Subacute thyroiditis is painful with a high ESR; post-partum thyroiditis affects about 5 percent of women in the year after delivery.
Too much levothyroxine or liothyronine
The commonest cause of a high free T4 in someone with known hypothyroidism. Weight loss, starting estrogen-free contraception, or stopping an interfering drug (calcium, iron, proton-pump inhibitors) can raise absorption and push a previously correct dose too high.
Biotin supplements (assay interference)
Doses of 5 to 10 mg or more a day, common in hair and nail supplements, make free T4 and free T3 read falsely high and TSH falsely low on many analyzers, mimicking Graves' disease. Stop biotin for at least 2 to 3 days (longer with high doses) before testing.
Amiodarone and iodine excess
Amiodarone contains 37 percent iodine and blocks T4 to T3 conversion, so free T4 runs high-normal or high in most people taking it; about 3 to 5 percent develop true amiodarone-induced thyrotoxicosis. Iodinated contrast and kelp supplements can also trigger hyperthyroidism in a nodular gland.
Excess hCG in early pregnancy or hyperemesis gravidarum
Human chorionic gonadotropin weakly stimulates the TSH receptor; in the first trimester free T4 can sit just above the non-pregnant range with a low TSH. It resolves by 16 to 20 weeks without treatment in most women.
Acute illness, heparin and some drugs
Heparin in the sample tube, high-dose salicylates, furosemide and some anticonvulsants displace T4 from binding proteins and raise measured free T4 transiently. Acute psychiatric illness can raise free T4 for the first 1 to 2 weeks.
TSH-secreting pituitary adenoma or thyroid hormone resistance
Both give a high free T4 with a TSH that is normal or raised rather than suppressed. They are rare (about 1 in a million for TSH-omas) and are diagnosed in specialist clinics with alpha-subunit measurement, genetic testing and pituitary MRI.

Symptoms that can go with it

Unintentional weight loss despite a normal or increased appetite
Fast or irregular heartbeat, palpitations, or new atrial fibrillation
Heat intolerance, sweating and warm, moist skin
Tremor of the hands, anxiety, irritability and difficulty sleeping
Frequent bowel movements or diarrhea
Muscle weakness, particularly in the thighs and upper arms
Lighter or missed periods; in Graves' disease, bulging or gritty eyes and a visible goiter
In older adults, often only fatigue, weight loss, depression or a new heart rhythm problem (apathetic hyperthyroidism)

Low free T4

A low free T4 with a raised TSH (usually above 10 mIU/L) is overt primary hypothyroidism: the thyroid gland cannot make enough hormone and the pituitary is pushing hard to compensate. In iodine-sufficient countries the usual cause is Hashimoto's (autoimmune) thyroiditis, which affects about 5 percent of adults and is 5 to 8 times more common in women. Other common causes are previous thyroid surgery, radioactive iodine treatment for Graves' disease, external radiation to the neck, and drugs such as lithium and amiodarone. Hypothyroidism develops slowly and the symptoms (fatigue, cold intolerance, weight gain, constipation, dry skin) are easy to attribute to something else.

A low free T4 with a TSH that is low, normal or only slightly raised is a different and more important pattern. The pituitary is failing to respond, which is called central (secondary) hypothyroidism. It is caused by pituitary tumors, pituitary surgery or radiation, head injury, Sheehan's syndrome after a difficult delivery, infiltrative diseases, and increasingly by immune checkpoint inhibitor cancer drugs. Because other pituitary hormones, including cortisol, are often affected, this pattern needs prompt specialist review, and cortisol must be checked and replaced before thyroid hormone is started.

In people on levothyroxine, a low free T4 with a raised TSH usually means the dose is too low or is not being absorbed: taking it with food, coffee, calcium, iron or a proton-pump inhibitor, or a new diagnosis of celiac disease, can each cut absorption substantially. A low free T4 can also be an artefact of severe acute illness (non-thyroidal illness or sick euthyroid syndrome), in which case thyroid tests are repeated after recovery rather than treated.

Common causes

Hashimoto's thyroiditis
Autoimmune destruction of thyroid tissue; thyroid peroxidase (TPO) antibodies are present in about 90 percent. Progression from subclinical to overt hypothyroidism runs at about 2 to 4 percent a year, faster when TPO antibodies are high.
Thyroid surgery or radioactive iodine treatment
Total thyroidectomy causes permanent hypothyroidism; after radioactive iodine for Graves' disease, 50 to 80 percent of people become hypothyroid within the first year and most of the rest eventually do.
Under-dosed or poorly absorbed levothyroxine
Full replacement is roughly 1.6 micrograms per kilogram a day. Calcium, iron, soy, coffee, sucralfate and PPIs taken near the dose, and gut conditions such as celiac disease or atrophic gastritis, can lower absorption by 20 to 40 percent.
Central hypothyroidism (pituitary or hypothalamic disease)
Pituitary adenomas, craniopharyngioma, pituitary surgery or radiotherapy, traumatic brain injury, Sheehan's syndrome and immunotherapy-related hypophysitis all reduce TSH drive. Free T4 is low with an inappropriately normal or low TSH.
Medications: lithium, amiodarone, tyrosine kinase inhibitors, checkpoint inhibitors
Lithium causes hypothyroidism in about 6 to 15 percent of users; amiodarone-induced hypothyroidism is more common in iodine-sufficient areas; sunitinib and related cancer drugs and the immune checkpoint inhibitors (pembrolizumab, nivolumab, ipilimumab) frequently cause thyroiditis followed by hypothyroidism.
Iodine deficiency or excess
Worldwide the most common cause of hypothyroidism is too little iodine; in the US it is rare but can occur with strict diets that avoid iodized salt, dairy and seafood. Paradoxically, large iodine loads (kelp supplements, contrast dye) can also shut hormone production down for weeks (the Wolff-Chaikoff effect).
Severe non-thyroidal illness
In intensive care, after major surgery or during prolonged fasting, free T4 on immunoassay can fall below normal with a normal or low TSH. This is adaptive and recovers; thyroid testing in hospital is best avoided unless thyroid disease is genuinely suspected.
Congenital hypothyroidism and thyroid dysgenesis
About 1 in 2,000 to 4,000 births; detected by newborn screening. Untreated it causes permanent intellectual disability, which is why screening within days of birth is universal in the US.
Phenytoin, carbamazepine and high-dose glucocorticoids
These lower measured free T4 by 20 to 40 percent on some assays by displacing T4 from binding proteins or speeding its clearance, usually without true hypothyroidism; TSH stays normal.

Symptoms that can go with it

Fatigue, sluggishness and needing more sleep than usual
Feeling cold when others are comfortable
Weight gain of a few kilograms, mainly fluid, despite no change in eating
Constipation, dry skin, brittle hair and hair thinning including the outer eyebrows
Slow heart rate, puffiness around the eyes and a hoarse voice
Low mood, slowed thinking and poor memory
Heavy or irregular periods, reduced fertility, and raised cholesterol on blood tests
Muscle aches, cramps and carpal tunnel symptoms

Read it alongside

TSH — the first-line test; the combination of TSH and free T4 places you in one of the four patterns (overt or subclinical hypo- or hyperthyroidism) or flags central disease.
Free T3 — ordered when TSH is low but free T4 is normal, because some hyperthyroidism (T3 toxicosis) raises T3 first; less useful in hypothyroidism.
Thyroid peroxidase (TPO) antibodies — positive in about 90 percent of Hashimoto's; predict progression from subclinical to overt hypothyroidism.
TSH-receptor antibodies (TRAb or TSI) — confirm Graves' disease when free T4 is high and TSH suppressed, and predict relapse after treatment.
Total cholesterol and LDL — rise in hypothyroidism and fall when free T4 is restored; a new high LDL is a reason to check thyroid function.
Morning cortisol — essential when central hypothyroidism is suspected, because untreated adrenal insufficiency can be dangerously unmasked by starting thyroid hormone.
Prolactin — mildly raised in primary hypothyroidism (TRH stimulates prolactin) and in pituitary disease.
Sodium and a CBC — hypothyroidism can cause low sodium and a mild anemia; both recover on treatment.

What usually happens next

If TSH is abnormal and free T4 is normal (subclinical disease), the usual step is a repeat test with TPO antibodies in 6 to 12 weeks, since a third or more of mildly abnormal TSH values normalise on their own.
If free T4 is low with a raised TSH, expect a discussion about starting levothyroxine, with a repeat TSH and free T4 at 6 to 8 weeks after any dose change.
If free T4 is high with a suppressed TSH, the next tests are TSH-receptor antibodies and, if those are negative, a radioactive iodine uptake scan or thyroid ultrasound to distinguish Graves' disease, a toxic nodule and thyroiditis.
If free T4 is low but TSH is not raised, ask specifically about pituitary disease: expect a morning cortisol, prolactin, LH, FSH and sex hormones, and usually a pituitary MRI.
Tell your clinician about biotin, kelp or iodine supplements, amiodarone, lithium, recent iodinated contrast, pregnancy, and when you took your levothyroxine relative to the blood draw.
If the free T4 and TSH do not fit together or do not fit how you feel, ask whether assay interference is possible and whether a free T4 by equilibrium dialysis would help.

Diet, lifestyle and natural levers with evidence

What can move FT4 once the cause is known. Nothing here replaces finding the cause.

Adequate (not excessive) iodine from diet

Strong evidence

Thyroxine is 65 percent iodine by weight, and the thyroid cannot make it without about 150 micrograms a day (220 to 250 in pregnancy). Iodized salt, dairy, eggs and seafood cover this for most Americans; people on strict plant-based diets who avoid iodized salt have intakes two to three times lower and higher rates of raised TSH in surveys. Correcting deficiency restores free T4 within weeks.

More is not better. Intakes above about 1,100 micrograms a day, and especially kelp supplements that can contain several milligrams per serving, can trigger either hypothyroidism or hyperthyroidism, particularly in people with Hashimoto's or nodules.

Selenium

Moderate evidence

Selenium is required by the deiodinase enzymes that convert T4 to T3 and by the antioxidant enzymes that protect thyroid tissue. Meta-analyses of randomised trials in Hashimoto's thyroiditis found that 200 micrograms a day of selenomethionine for 3 to 12 months lowered TPO antibodies by about 20 to 40 percent, with small or no change in free T4 or TSH in people already taking levothyroxine. In Graves' orbitopathy, 6 months of selenium improved eye symptoms and quality of life in a European trial.

The upper limit is 400 micrograms a day; chronic excess causes hair loss, nail changes and nerve symptoms. Average US dietary intake is already around 100 micrograms, and one Brazil nut contains 70 to 90 micrograms.

Take levothyroxine consistently and away from food, coffee, calcium and iron

Strong evidence

Taking levothyroxine on an empty stomach 30 to 60 minutes before breakfast, or at bedtime at least 3 hours after the last meal, increases absorption and lowers TSH by roughly 1 to 1.5 mIU/L compared with taking it with food in crossover trials. Separating it from calcium, iron, soy and antacids by at least 4 hours avoids a 20 to 40 percent drop in absorption. Consistency matters more than the exact time.

Treat celiac disease and correct iron, vitamin D and B12 deficiency

Moderate evidence

Autoimmune thyroid disease travels with celiac disease (2 to 5 percent), pernicious anemia and iron deficiency. Iron is needed by thyroid peroxidase, and in a trial of iron-deficient women, correcting iron improved the response to iodine. In people with celiac disease, a gluten-free diet lowered the levothyroxine dose needed by about 20 to 30 percent as the gut healed. Low vitamin D is associated with higher TPO antibodies, though supplement trials show only small antibody reductions and no consistent change in free T4.

Iron and calcium tablets must still be taken at least 4 hours away from levothyroxine.

Stop smoking

Strong evidence

Smoking roughly doubles the risk of Graves' disease and increases the risk of Graves' eye disease 2 to 7 fold, with heavier smokers at highest risk; it also worsens the response to eye-disease treatment. Quitting reduces the risk of relapse after antithyroid drugs. In hypothyroidism, smoking has a smaller effect but thiocyanate from smoke competes with iodine uptake.

Avoid high-dose biotin before testing

Strong evidence

This does not change your thyroid, but it changes your result. Case series and FDA safety communications document falsely high free T4 and falsely low TSH with biotin doses of 5 mg a day or more, leading to misdiagnosis of Graves' disease and unnecessary treatment. Stopping biotin for 2 days normalises results on most assays; a week is safer after very high doses.

When to see a clinician promptly

A very high free T4 with fever above 38.5 C (101 F), heart rate over 130, agitation, confusion, vomiting or jaundice: possible thyroid storm, which is life-threatening and needs emergency care.
A very low free T4 with a body temperature below 35 C (95 F), slow heart rate, drowsiness or confusion, especially in an older person in winter or after an infection: possible myxedema coma, an emergency.
Low free T4 with a TSH that is not raised, particularly with headache, visual field loss, dizziness on standing, loss of periods or low libido: possible pituitary disease; cortisol must be checked before thyroid hormone is started.
High free T4 with a new fast or irregular heartbeat, chest pain or breathlessness, or any abnormal thyroid result in pregnancy or when planning pregnancy.
Sudden painful swelling in the front of the neck with fever (subacute thyroiditis), or a neck lump that is growing, hard or associated with hoarseness or difficulty swallowing.
Symptoms of over- or under-replacement on levothyroxine (palpitations, tremor and weight loss, or fatigue, cold intolerance and weight gain), which warrant a dose review rather than waiting for the next routine test.

Frequently asked questions

What is a normal free T4 level?

On most US immunoassays the adult reference range is about 0.8 to 1.8 ng/dL (10 to 23 pmol/L), but individual labs use slightly different ranges such as 0.82 to 1.77 or 0.9 to 1.7 ng/dL, so compare your result with the range printed on your report. Healthy people have a narrow personal set point within that range, so a change from 1.5 to 0.9 can be meaningful even though both are 'normal', especially if TSH has moved the other way.

What does low free T4 with normal TSH mean?

Three things, in order of likelihood: the assay is being pulled down by illness or a drug (phenytoin, carbamazepine, high-dose steroids, heparin), you are in the early recovery phase of a severe illness, or you have central hypothyroidism from a pituitary or hypothalamic problem. Central hypothyroidism is uncommon but important because it often comes with low cortisol and other pituitary hormone deficits, so a repeat test and, if it persists, a morning cortisol and pituitary assessment are the usual next steps.

What does high free T4 with normal TSH mean?

The commonest explanation is biotin in a supplement interfering with the assay, followed by taking a levothyroxine tablet shortly before the blood draw, amiodarone treatment, and antibodies in your blood that cross-react with the test. Rarely it is a TSH-secreting pituitary tumor or a genetic resistance to thyroid hormone. Stop biotin, repeat the test taking your tablet afterwards, and if the pattern persists ask for free T4 by equilibrium dialysis and an endocrinology opinion.

Is a free T4 of 0.7 low? Is 1.9 high?

A free T4 of 0.7 ng/dL is just below most reference ranges; with a TSH above 10 it indicates hypothyroidism, with a normal TSH it may be a lab effect or early central disease and should be repeated. A free T4 of 1.9 ng/dL is just above most ranges; with a TSH under 0.1 it indicates mild hyperthyroidism or over-replacement, with a normal TSH it is most often biotin or timing of a levothyroxine dose. The TSH and your symptoms decide what a borderline value means.

What is the difference between free T4 and total T4?

Total T4 measures all the thyroxine in your blood, 99.97 percent of which is bound to proteins and inactive. Free T4 measures only the unbound, active fraction. Total T4 rises with estrogen (pregnancy, oral contraceptives, HRT) and falls with low protein states and some drugs without any real change in thyroid function, which is why free T4 has largely replaced it. Total T4 with a T3 uptake (free thyroxine index) is an older way of estimating the same thing.

Should free T4 be checked with TSH, or is TSH enough?

For screening a healthy adult, TSH alone is enough and is what most guidelines recommend. Free T4 is added when TSH is abnormal, when pituitary disease is possible, in pregnancy, when adjusting treatment for hyperthyroidism (TSH can stay suppressed for months after free T4 normalises), and when symptoms strongly suggest thyroid disease despite a normal TSH. Adding free T4 to every TSH test mostly generates borderline values that need repeating.

Can free T4 be low in pregnancy?

Yes, and it is often not a problem. In the second and third trimesters, binding proteins rise and most immunoassays read free T4 10 to 30 percent lower than the non-pregnant range, so a value of 0.7 ng/dL with a normal TSH is usually fine. The American Thyroid Association recommends trimester-specific ranges or, if the lab does not have them, interpreting TSH as the primary test. A genuinely low free T4 with a raised TSH in pregnancy does need treatment because thyroid hormone is essential for fetal brain development.

How long does it take free T4 to change after starting or adjusting levothyroxine?

Free T4 reaches a new steady state in about 2 to 3 weeks because the hormone's half-life is around 7 days, but TSH lags and takes 6 to 8 weeks to settle, which is why repeat testing is scheduled 6 to 8 weeks after a dose change. Symptoms improve over the same 4 to 8 weeks. If you test sooner, free T4 will reflect the new dose but TSH will not yet.

References

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Hypothyroidism (underactive thyroid). Mayo Clinic (2024)View source
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Ross DS, Burch HB, Cooper DS, et al.. 2016 American Thyroid Association guidelines for diagnosis and management of hyperthyroidism and other causes of thyrotoxicosis. Thyroid (2016)
Alexander EK, Pearce EN, Brent GA, et al.. 2017 guidelines of the American Thyroid Association for the diagnosis and management of thyroid disease during pregnancy and the postpartum. Thyroid (2017)
Chaker L, Bianco AC, Jonklaas J, Peeters RP. Hypothyroidism. The Lancet (2017)
Li D, Radulescu A, Shrestha RT, et al.. Biotin interference in clinical immunoassays: a cause for concern. JAMA (2017)
The FDA warns that biotin may interfere with lab tests: FDA safety communication. US Food and Drug Administration (2019)
Overview of thyroid function. Merck Manual Professional Version (2024)
van Zuuren EJ, Albusta AY, Fedorowicz Z, Carter B, Pijl H. Selenium supplementation for Hashimoto's thyroiditis: summary of a Cochrane systematic review. European Thyroid Journal (2014)
Bach-Huynh TG, Nayak B, Loh J, Soldin S, Jonklaas J. Timing of levothyroxine administration affects serum thyrotropin concentration. Journal of Clinical Endocrinology and Metabolism (2009)
Thyroid function tests. American Thyroid Association (2023)View source

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This page is educational and is not medical advice or a diagnosis. Lab results have to be interpreted together with your history, symptoms and other results by a clinician who knows you. Do not start or stop a supplement to change a lab value before the cause is known.