Lab result explainedThyroidLast updated November 10, 2026

Free T3 blood test: normal range, what low and high free T3 mean, and T3 vs T4

Free triiodothyronine (free T3) (FT3) · Thyroid panel (TSH, free T4, free T3) · reported in pg/mL (SI: pmol/L; pg/mL x 1.536 = pmol/L)

Free T3 (free triiodothyronine) is the unbound, biologically active form of thyroid hormone circulating in your blood, and a free T3 blood test measures the tiny fraction (about 0.3 percent of total T3) that is not attached to carrier proteins and is available to enter cells. The normal adult range is about 2.3 to 4.2 pg/mL (some labs 2.0 to 4.4 pg/mL; 3.5 to 6.5 pmol/L in SI units), with ranges differing by lab and assay. T3 is the hormone that actually sets your metabolic rate, heart rate, body temperature and energy use; T4 is mostly a reservoir that tissues convert into T3 as needed.

The thyroid gland makes mostly T4 and only about 20 percent of the body's T3 directly. The other 80 percent is produced in the liver, kidneys, muscle and brain by deiodinase enzymes that remove one iodine atom from T4. This matters for reading the test: in early hypothyroidism the failing gland and the deiodinases favor T3 production, so free T3 stays normal long after TSH has risen and free T4 has fallen, which makes free T3 a poor test for an underactive thyroid. In hyperthyroidism the opposite happens, and free T3 rises earlier and higher than free T4.

The most common reason for a low free T3 is not thyroid disease at all. Any significant illness, surgery, calorie restriction or fasting switches off the conversion of T4 to T3 within a day or two, a protective energy-saving response called non-thyroidal illness (or low T3 syndrome), with a normal TSH. Professional guidelines therefore recommend TSH and free T4 to diagnose and monitor hypothyroidism, and reserve free T3 for suspected hyperthyroidism, where it identifies the 5 to 10 percent of cases (T3 toxicosis) in which free T4 is still normal.

FT3 at a glance

What it is: the unbound, active thyroid hormone; about 80 percent is made from T4 in the liver, kidney and other tissues rather than by the thyroid itself.
Normal adult range: about 2.3-4.2 pg/mL (2.0-4.4 in some labs; 3.5-6.5 pmol/L). Labs and assays differ; children and teenagers run higher, older adults slightly lower.
High free T3 with a low TSH (under 0.1-0.4 mIU/L): hyperthyroidism, including T3 toxicosis where free T4 is still normal. Biotin supplements can fake this pattern.
Low free T3 with a normal TSH: usually illness, fasting, crash dieting or medication (non-thyroidal illness), not hypothyroidism. Low free T3 with a high TSH: established hypothyroidism.
T3 vs T4: T4 is the reservoir hormone the thyroid makes most of; T3 is the active hormone cells use. TSH and free T4 are the standard tests; free T3 is added mainly when hyperthyroidism is suspected.
Next step: interpret with TSH and free T4 together; a lone abnormal free T3 in someone who feels well is usually repeated in 6 to 8 weeks when well and off biotin.

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What FT3 measures

Over 99.7 percent of the T3 in blood is bound to thyroxine-binding globulin, transthyretin and albumin. Only the free fraction can cross into cells, bind thyroid hormone receptors in the nucleus and switch on the genes that control oxygen consumption, heat production, heart rate, cholesterol clearance, bone turnover and brain function. The free T3 assay estimates this unbound fraction directly with an immunoassay; a total T3 test measures bound plus free and is affected by anything that changes binding proteins, such as estrogen, pregnancy and liver disease. Free T3 is reported in pg/mL in the US (pmol/L elsewhere; pg/mL x 1.536 = pmol/L).

T3 has a short half-life of about one day compared with a week for T4, so free T3 responds quickly to changes in conversion. Three deiodinase enzymes control the supply: type 1 and type 2 generate T3 from T4, and type 3 inactivates T4 and T3 into reverse T3 and T2. Illness, starvation, cortisol, inflammatory cytokines, beta-blockers, amiodarone and some other drugs shift the balance toward inactivation, lowering free T3 within 24 to 48 hours; the TSH does not rise, because the brain's own deiodinase keeps its local T3 supply adequate.

Free T3 immunoassays are the least reliable of the thyroid tests. Results vary between manufacturers by 10 to 20 percent, so the same person can be normal on one platform and slightly low on another, and interference from biotin (vitamin B7) at doses of 5 mg or more a day, from heterophile antibodies and from some rheumatoid factors can produce falsely high or low readings. The test is usually drawn in the morning; levels show a small daily rhythm and are not affected by food. For people on levothyroxine, timing relative to the dose matters less for free T3 than for free T4, but a sample taken within a few hours of liothyronine (T3) tablets will be transiently high.

Normal range for FT3

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

Group	Typical range (pg/mL (SI: pmol/L; pg/mL x 1.536 = pmol/L))	Notes
Adults, typical reference range	About 2.3-4.2 pg/mL (3.5-6.5 pmol/L)	Labs differ: 2.0-4.4, 2.3-4.1 and 2.8-4.4 pg/mL are all in use. Use the range on your report.
Children and adolescents	Higher, roughly 3.0-5.5 pg/mL depending on age	Free T3 is highest in infancy and falls through the teenage years; age-specific ranges apply.
Adults over 65-70	Slightly lower, often 2.0-3.8 pg/mL	Conversion of T4 to T3 declines modestly with age; a low-normal value in an older adult with a normal TSH is usually fine.
Pregnancy	Lower in the second and third trimesters	Assay interference from binding proteins makes free T3 unreliable in pregnancy; TSH and free T4 (or total T3 with a 1.5x adjusted range) are used.
Overt hyperthyroidism	Above the upper limit, commonly 5-15 pg/mL, with TSH under 0.1 mIU/L	Free T3 is often raised 2 to 4 fold in Graves' disease and toxic nodules; the ratio of total T3 to T4 above 20 (ng/dL to ug/dL) favors Graves'.
T3 toxicosis	High free T3, normal free T4, TSH under 0.1 mIU/L	About 5-10 percent of hyperthyroidism, especially early Graves' disease and toxic nodules; the reason free T3 is checked when TSH is low and free T4 normal.
Non-thyroidal illness (low T3 syndrome)	Low free T3 with a normal TSH and normal or low free T4	Hospitalized and acutely ill people, after surgery, with fasting or very low-calorie diets. Recovers over weeks once the illness or diet ends; not treated with thyroid hormone.
Hypothyroidism	Low-normal or low free T3 with TSH above 4.5-10 mIU/L and low free T4	Free T3 is the last of the three to fall and is often still normal; it is not used to diagnose or monitor hypothyroidism.
On levothyroxine	Often low-normal free T3 (2.3-3.0 pg/mL) with high-normal free T4	Expected, because the T3 must all be made from the T4 dose; a free T3 in the lower part of the range with a normal TSH is a treated, not under-treated, state.

High free T3

A high free T3 together with a suppressed TSH (under 0.1 mIU/L, or under 0.4 in milder cases) means hyperthyroidism: the body is being exposed to too much active thyroid hormone. The usual causes are Graves' disease (an autoimmune antibody that drives the thyroid), one or more overactive thyroid nodules, the early phase of thyroiditis when stored hormone leaks out, and too high a dose of thyroid medication. If free T4 is also high the picture is classic hyperthyroidism; if free T4 is normal it is T3 toxicosis, which behaves the same way and is treated the same way.

A free T3 that is high while TSH is normal or high is a different and much rarer situation, and the first explanation to exclude is a laboratory artefact. Biotin at the 5 to 10 mg doses found in hair, skin and nail supplements interferes with many thyroid assays and makes free T3 and free T4 read high and TSH read low; stopping it for 2 to 3 days (longer at higher doses) and repeating the test resolves it. Heterophile antibodies and some anti-T3 autoantibodies can do the same. Genuine causes of this pattern are uncommon: a TSH-secreting pituitary tumor or inherited resistance to thyroid hormone, both needing specialist assessment.

In someone taking liothyronine (Cytomel) or desiccated thyroid (Armour, NP Thyroid), a free T3 drawn within 4 to 6 hours of a dose is expected to be above range and does not by itself mean over-treatment; the TSH and a trough sample taken before the dose are used to judge the dose. Mild, isolated elevations of free T3 with a normal TSH in a well person are often assay noise and are simply repeated.

Common causes

Graves' disease
Antibodies against the TSH receptor stimulate the whole gland. Free T3 rises disproportionately (the gland makes relatively more T3), TSH is suppressed below 0.01-0.1 mIU/L, and TSH-receptor antibodies (TRAb) are positive. The commonest cause of hyperthyroidism under 50.
Toxic nodule or toxic multinodular goitre
One or several nodules make hormone independently of TSH. More common over 60 and in iodine-poor regions; T3 toxicosis (high free T3 with a normal free T4) is particularly typical of nodules.
Thyroiditis (subacute, postpartum, silent, drug-induced)
Inflammation releases stored hormone for 2 to 8 weeks, raising free T3 and free T4 with a low TSH, then often swinging to hypothyroidism before recovery. Radioactive iodine uptake is low, which distinguishes it from Graves' disease.
Too much thyroid medication
Over-replacement with levothyroxine raises free T4 first and free T3 modestly; liothyronine or desiccated thyroid raises free T3 sharply for several hours after each dose. Some weight-loss and 'thyroid support' supplements have been found to contain unlabeled T3.
Biotin and other assay interference
High-dose biotin (5 mg a day or more) produces a false pattern of high free T3, high free T4 and low TSH on streptavidin-based immunoassays. Heterophile antibodies and anti-T3 autoantibodies can also give spurious results; a different assay method confirms.
Iodine excess
Amiodarone, iodinated contrast dye, kelp or iodine supplements can trigger hyperthyroidism in people with nodules or latent Graves' disease (Jod-Basedow effect), weeks to months after exposure.
Pregnancy-related and hCG-driven hyperthyroidism
High hCG in early pregnancy, hyperemesis gravidarum and molar pregnancy stimulate the TSH receptor, raising free T3 and T4 transiently in the first trimester.
TSH-secreting pituitary adenoma and thyroid hormone resistance
Rare causes of high free T3 and free T4 with a TSH that is normal or raised rather than suppressed. Thyroid hormone resistance is inherited and the person is usually not clinically hyperthyroid.
Struma ovarii and metastatic thyroid cancer
Very rare: thyroid tissue in an ovarian tumor, or large amounts of functioning metastatic follicular thyroid cancer, produce hormone outside the neck.

Symptoms that can go with it

Fast or pounding heartbeat, palpitations or a new irregular rhythm (atrial fibrillation), especially over 60
Weight loss despite a normal or increased appetite
Heat intolerance, sweating, warm moist skin
Anxiety, irritability, restlessness, difficulty sleeping and a fine tremor of the hands
Frequent bowel movements or diarrhea
Muscle weakness, particularly in the thighs and upper arms, and tiredness
Lighter or missed periods; bulging or gritty eyes in Graves' disease; an enlarged thyroid (goitre)
Confusion, fever, very fast heart rate or heart failure, which can signal thyroid storm

Low free T3

A low free T3 means less active thyroid hormone is reaching the blood, and the TSH tells you why. If the TSH is normal, the most likely explanation is that your body has deliberately reduced the conversion of T4 to T3: this happens within 24 to 48 hours of a significant infection, surgery, heart attack, fasting, a very low-calorie diet, uncontrolled diabetes, severe depression or anorexia, and with drugs such as high-dose steroids, amiodarone and propranolol. This is non-thyroidal illness or low T3 syndrome. It is adaptive, the thyroid itself is healthy, it resolves as the illness or diet ends, and randomised trials in sick patients have not shown benefit from giving T3, so it is not treated.

If the TSH is high (above 4.5 to 10 mIU/L) and free T4 is low as well, the low free T3 is part of established hypothyroidism, usually from Hashimoto's thyroiditis, thyroid surgery, radioactive iodine or iodine deficiency; free T3 falls late in the process, so by the time it is low the hypothyroidism is usually clear from TSH and free T4 alone. If TSH is low or inappropriately normal with low free T4 and free T3, the problem may be the pituitary (central hypothyroidism), which needs specialist assessment.

Many people find a low or low-normal free T3 on a panel ordered while they feel tired, often while taking levothyroxine, and wonder whether it explains their symptoms. On levothyroxine, a free T3 in the lower third of the range with a normal TSH is the expected state, because every molecule of T3 has to be made from the tablet's T4. Trials comparing levothyroxine alone with levothyroxine plus T3 have, on average, shown no difference in symptoms, quality of life or cognition, though a minority of people report feeling better on combinations and guidelines allow a supervised trial in selected cases. A low free T3 in a well person with normal TSH and free T4 is not a diagnosis and is usually repeated when well.

Common causes

Non-thyroidal illness (low T3 syndrome)
Any acute or chronic illness, surgery, trauma, burns, sepsis, heart failure, kidney failure, liver disease or cancer suppresses deiodinase activity; free T3 falls, reverse T3 rises, TSH stays normal. Found in up to 70 percent of hospital inpatients and resolves with recovery.
Fasting, very low-calorie dieting and anorexia nervosa
Within 2 to 3 days of eating under about 800 kcal a day, T3 production drops by 20 to 50 percent to conserve energy. Chronic under-eating and anorexia keep it low; refeeding restores it over weeks.
Hypothyroidism (Hashimoto's, after surgery or radioiodine, iodine deficiency)
Free T3 is the last to fall. By the time it is low, TSH is usually well above 10 mIU/L and free T4 is low; TPO antibodies are positive in Hashimoto's.
Treatment with levothyroxine (T4 only)
Free T3 commonly sits in the lower part of the range with a high-normal free T4, because conversion is the only source of T3. This is the expected profile, not a deficiency, if TSH is normal.
Medications that block conversion
Glucocorticoids (prednisone, dexamethasone), amiodarone, propranolol at high doses, propylthiouracil, some iodinated contrast agents and the antiepileptics phenytoin and carbamazepine lower free T3.
Central hypothyroidism
Pituitary or hypothalamic disease (tumors, surgery, radiotherapy, Sheehan syndrome, head injury) reduces TSH, so T4 and T3 fall with a TSH that is low or misleadingly normal.
Selenium or iodine deficiency
Deiodinases are selenium-dependent enzymes; severe selenium deficiency (rare in the US) impairs T4-to-T3 conversion, and iodine deficiency limits hormone production overall.
Older age
Conversion declines modestly with age; a free T3 at or just below the lower limit with a normal TSH in someone over 70 is usually physiological.
Overtraining and severe sleep deprivation
Prolonged very heavy training without adequate calories, and chronic sleep loss, lower T3 modestly as part of the same energy-conserving response.

Symptoms that can go with it

Often none, particularly when the cause is illness or dieting and the TSH is normal
Tiredness, low mood and slowed thinking
Feeling cold, dry skin, hair thinning and constipation when true hypothyroidism is present
Weight gain or difficulty losing weight, and fluid retention or puffiness
Slow heart rate and muscle aches or cramps
Heavier or irregular periods, reduced libido
Very low body temperature, confusion and drowsiness in severe untreated hypothyroidism (myxedema)

Read it alongside

TSH — the master test; a suppressed TSH makes a high free T3 meaningful, and a normal TSH makes a low free T3 almost always non-thyroidal.
Free T4 — high free T4 with high free T3 is classic hyperthyroidism; normal free T4 with high free T3 is T3 toxicosis; low free T4 with low free T3 and high TSH is hypothyroidism.
Reverse T3 — rises when illness or fasting diverts T4 away from T3; a high reverse T3 with low free T3 confirms non-thyroidal illness, although it is rarely needed.
TSH-receptor antibodies (TRAb) and TPO antibodies — identify Graves' disease (TRAb) and Hashimoto's thyroiditis (TPO) as the cause of abnormal hormone levels.
Total T3 — preferred over free T3 in pregnancy and when binding proteins are abnormal; a total T3 to total T4 ratio above 20 suggests Graves' disease rather than thyroiditis.
CRP, albumin and a basic metabolic panel — markers of the illness that is usually behind a low free T3 with normal TSH.
Cortisol — low cortisol (Addison's disease) can coexist with thyroid disease and must be treated first; high cortisol lowers T3 conversion.
Heart rate and ECG — atrial fibrillation is the most important complication of high free T3, particularly over 60.

What usually happens next

A high free T3 with a low TSH is usually followed by TSH-receptor antibodies and, if needed, a radioactive iodine uptake scan or thyroid ultrasound to distinguish Graves' disease, nodules and thyroiditis, because they are treated differently.
A high free T3 with a normal TSH is repeated off biotin (stop for at least 2 to 3 days, longer at doses over 10 mg) and, if persistent, checked on a different assay before rarer causes are considered.
A low free T3 with a normal TSH in someone who has been unwell, dieting or on steroids is repeated 6 to 8 weeks after recovery; no thyroid treatment is started on this result alone.
A low free T3 with a high TSH is managed as hypothyroidism using TSH and free T4; free T3 does not need to be rechecked routinely.
If you take liothyronine or desiccated thyroid, ask for blood to be drawn before your morning dose so the result is interpretable.
If you feel unwell on levothyroxine with a normal TSH and low-normal free T3, discuss dose timing, absorption (iron, calcium, coffee, acid suppressants), other causes of fatigue and, in selected cases, a supervised trial of combination therapy.

Diet, lifestyle and natural levers with evidence

What can move FT3 once the cause is known. Nothing here replaces finding the cause.

Adequate calories and avoiding crash diets

Moderate evidence

Calorie restriction is the best-documented lifestyle influence on free T3. In controlled studies, fasting or eating under about 800 kcal a day lowered serum T3 by 20 to 50 percent within days while TSH stayed normal, and long-term calorie restrictors have T3 levels around 20 percent below matched controls. Restoring intake, particularly carbohydrate, brings T3 back over one to three weeks. There is no evidence that any diet raises free T3 above a person's normal in someone eating adequately.

Iodine sufficiency (not excess)

Moderate evidence

The thyroid needs about 150 mcg of iodine a day (220 to 290 mcg in pregnancy and breastfeeding) to make T4 and T3; correcting deficiency normalizes hormone output and shrinks goitre. In iodine-sufficient populations such as the US, extra iodine does not raise free T3 and high doses (kelp, 'thyroid support' blends with milligram amounts) can trigger both hyper- and hypothyroidism in susceptible people.

Avoid kelp and high-dose iodine supplements if you have nodules, Graves' disease or Hashimoto's; do not take iodine in an attempt to raise free T3.

Selenium

Preliminary evidence

Deiodinases are selenoproteins, and meta-analyses show selenium (100 to 200 mcg a day for 3 to 12 months) lowers TPO antibody levels in Hashimoto's thyroiditis by roughly 10 to 25 percent, but trials have not shown a consistent change in free T3, free T4 or TSH in people who are selenium-replete, and the US diet generally supplies enough.

More than 400 mcg a day can cause selenium toxicity (hair loss, nail changes, nausea); long-term supplementation has been associated with a higher risk of type 2 diabetes in one large trial.

Treating the underlying illness, sleep loss or overtraining

Moderate evidence

Because most low free T3 results reflect non-thyroidal illness, the lever that reliably restores it is recovery: free T3 returns to baseline over 2 to 6 weeks after an infection or operation, and within weeks of restoring sleep and reducing training load in overtrained athletes. Randomised trials of giving T3 during critical illness or after heart surgery have not improved outcomes, which is why the number is left to correct itself.

Biotin timing and medication review

Strong evidence

This does not change your thyroid, but it changes the result. Stopping biotin supplements of 5 mg or more for at least 2 to 3 days before testing (a week for 10 mg or more) removes the falsely high free T3 and low TSH pattern seen on many assays; taking liothyronine or desiccated thyroid after rather than before the blood draw prevents a transient peak being read as over-treatment. Tell the lab about steroids, amiodarone, propranolol and antiepileptics, which lower free T3.

When to see a clinician promptly

A very fast or irregular heartbeat, chest pain, breathlessness, fever with agitation or confusion in someone with a high free T3: possible atrial fibrillation or thyroid storm, which is life-threatening and needs emergency care.
Severe drowsiness, confusion, a body temperature below about 95 F (35 C) or a very slow pulse in someone with known or suspected hypothyroidism: possible myxedema coma, an emergency.
A high free T3 with a suppressed TSH, for antibody tests and a plan; untreated hyperthyroidism damages the heart and bones within months.
Any abnormal free T3 in pregnancy or while trying to conceive, because both over- and under-active thyroid affect the pregnancy and need specialist interpretation.
A low free T3 with a low or normal TSH and low free T4, which can indicate a pituitary problem.
New eye symptoms (bulging, double vision, pain) with hyperthyroidism, which need early assessment for Graves' eye disease.

Frequently asked questions

What is a normal free T3 level?

For adults, about 2.3 to 4.2 pg/mL on most US assays (3.5 to 6.5 pmol/L), with some labs using 2.0 to 4.4 or 2.8 to 4.4. Children run higher and adults over 70 slightly lower. Because free T3 assays differ between manufacturers by 10 to 20 percent, always compare your result with the range on your own report rather than a generic one, and read it together with your TSH and free T4.

What does a low free T3 with a normal TSH mean?

Almost always that your body has reduced conversion of T4 to T3 because of something other than the thyroid: a recent illness or operation, dieting or fasting, uncontrolled diabetes, depression, steroid or beta-blocker medication, or being on levothyroxine, which naturally gives a low-normal free T3. This is called non-thyroidal illness or low T3 syndrome. It is not hypothyroidism, it is not treated with thyroid hormone, and the test is usually repeated 6 to 8 weeks after you are well.

What does a high free T3 mean?

With a low TSH, hyperthyroidism, most often Graves' disease, an overactive nodule, thyroiditis or too much thyroid medication. With a normal TSH, the first thing to rule out is biotin or another assay interference; genuine causes of that pattern (a TSH-producing pituitary tumor or thyroid hormone resistance) are rare. If you take liothyronine or desiccated thyroid, a sample within a few hours of the dose will be high and is not meaningful.

What is the difference between T3 and T4?

T4 (thyroxine) has four iodine atoms and is the main hormone the thyroid makes, about 80 to 90 percent of its output; it acts as a circulating reservoir with a one-week half-life. T3 (triiodothyronine) has three iodine atoms, is three to four times more potent, has a one-day half-life and is the form that binds receptors inside cells; most of it is made from T4 in the liver, kidney and other tissues. Levothyroxine is T4; liothyronine is T3. For diagnosing thyroid problems, TSH and free T4 are the primary tests and free T3 is added when hyperthyroidism is suspected.

Can I have hypothyroidism with a normal TSH but low free T3?

Primary hypothyroidism, the common type, raises TSH before anything else, so a normal TSH with low free T3 is not primary hypothyroidism. The rare exception is central hypothyroidism from pituitary or hypothalamic disease, where TSH is low or inappropriately normal and free T4 is also low. If free T4 is normal, a low free T3 with a normal TSH is non-thyroidal illness, medication, dieting or assay variation rather than an underactive thyroid.

Should I ask for T3 (liothyronine) if my free T3 is low on levothyroxine?

Most people on levothyroxine have a free T3 in the lower part of the range with a normal TSH, and that is the expected profile. Randomised trials comparing levothyroxine alone with levothyroxine plus liothyronine have, on average, found no difference in symptoms, mood, cognition or quality of life, although some individuals prefer the combination and a 2021 consensus from the American, European and British thyroid associations allows a carefully monitored trial in people with persistent symptoms despite a normal TSH. It is a conversation to have with your clinician, not something to self-treat with supplements or desiccated thyroid bought online.

Does biotin affect free T3?

Yes, on many laboratory platforms. Biotin at the 5 to 10 mg doses in hair, skin and nail products (far above the 30 mcg daily requirement) interferes with the assay chemistry and produces a falsely high free T3 and free T4 with a falsely low TSH, mimicking Graves' disease. Stop biotin for at least 2 to 3 days before a thyroid test, or a week at doses of 10 mg or more, and tell the lab you were taking it.

Do I need to fast for a free T3 test?

No, food does not affect it. Morning samples are preferred because levels and TSH have a mild daily rhythm. If you take liothyronine or desiccated thyroid, have the blood drawn before your dose; levothyroxine can be taken after the draw too, mainly to keep free T4 interpretable. Mention recent illness, dieting, steroids and biotin, all of which change the result.

References

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Bianco AC, Dumitrescu A, Gereben B, et al.. Paradigms of dynamic control of thyroid hormone signaling. Endocrine Reviews (2019)
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Wichman J, Winther KH, Bonnema SJ, Hegedus L. Selenium supplementation significantly reduces thyroid autoantibody levels in patients with chronic autoimmune thyroiditis: a systematic review and meta-analysis. Thyroid (2016)
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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.