Thyroid Test

TSH Test (Thyroid Stimulating Hormone)

TSH is the single most important thyroid test. It acts like a thermostat for your thyroid: when thyroid function is off, TSH rises or falls. Here's what your result means.

Written and clinically reviewed by Suman Konda, PharmD, Clinical Pharmacist · Based on peer-reviewed sources · Editorial policy · Not medical advice

Last reviewed and updated: 01 August 2026 · How we check our content

What is TSH?

TSH (Thyroid Stimulating Hormone) is produced by the pituitary gland in the brain. It tells the thyroid gland how much thyroid hormone (T3 and T4) to produce. TSH works like a thermostat: if thyroid hormone levels are low, the pituitary pumps out more TSH to stimulate the thyroid. If thyroid hormone is too high, TSH drops to signal the thyroid to slow down. This is why TSH moves in the opposite direction to thyroid hormones.

TSH Normal Range

Group	Normal TSH (mIU/L)
Adults (general)	0.4 – 4.0 mIU/L
Adults (many labs)	0.5 – 5.0 mIU/L
Pregnancy (1st trimester)	0.1 – 2.5 mIU/L
Pregnancy (2nd trimester)	0.2 – 3.0 mIU/L
Pregnancy (3rd trimester)	0.3 – 3.0 mIU/L
Elderly (>70 yrs)	Up to 6.0 may be acceptable

TSH Test (Thyroid Stimulating Hormone): reference intervals by group (mIU/L), drawn from the table above. Intervals differ between laboratories: compare your result against the range printed on your own report.

What does HIGH TSH mean?

HIGH TSH above 4.0 mIU/L: Hypothyroidism

High TSH means your thyroid is underperforming: the pituitary is shouting louder to try to get the sluggish thyroid to produce more hormone. This is hypothyroidism (underactive thyroid). Most common cause: Hashimoto's thyroiditis (autoimmune attack on the thyroid). Symptoms: fatigue, weight gain, feeling cold, constipation, hair loss, dry skin, depression, heavy periods, brain fog. Treatment: levothyroxine tablet once daily, usually lifelong. Most people feel dramatically better within 4–8 weeks of starting treatment.

What does LOW TSH mean?

LOW TSH below 0.4 mIU/L: Hyperthyroidism

Low TSH means your thyroid is overproducing hormones: the pituitary has gone quiet because there's already too much thyroid hormone. This is hyperthyroidism (overactive thyroid). Most common cause: Graves' disease (autoimmune), toxic nodular goitre, or excess thyroid medication. Symptoms: weight loss despite increased appetite, palpitations (fast/irregular heartbeat), anxiety, trembling hands, heat intolerance, sweating, diarrhoea, bulging eyes (Graves'). Requires prompt treatment, untreated hyperthyroidism can cause atrial fibrillation and bone loss.

Scroll sideways to see the whole diagram

How TSH and free T4 combine to give a thyroid diagnosis.

What the grid shows. TSH and free T4 are interpreted together. A high TSH with a low free T4 is overt hypothyroidism; a high TSH with a normal free T4 is subclinical hypothyroidism, which is the commonest abnormal pattern and often needs monitoring rather than treatment. A low TSH with a high free T4 is overt hyperthyroidism, and a low TSH with a normal free T4 is the subclinical form. The one combination that points away from the thyroid itself is a low TSH with a low free T4, which suggests a pituitary cause. The counter-intuitive part is that a high TSH means an underactive gland, because TSH is the signal telling the thyroid to work harder.

Normal TSH levels by age

The upper limit of the TSH reference interval is not fixed across adult life: it rises steadily with age. This matters because a value that would be called abnormal in a 25-year-old can be entirely normal at 75, and applying one adult range to everyone is a recognised cause of overdiagnosis in older people.

In a cross-sectional analysis of 8,308 US adults from NHANES, the 97.5th percentile of TSH rose from 3.56 mIU/L in people aged 20 to 29 to 7.49 mIU/L in those over 80. A Scottish population study of people with no known thyroid disease found the same direction of effect, with the 97.5th centile rising from 3.98 to 5.94 mU/L across the adult age span.

Age group	Approximate upper limit (97.5th percentile)	What it means in practice
20–29 years	~3.6 mIU/L	The conventional 4.0 mIU/L cut-off is, if anything, slightly generous at this age
30–49 years	~4.0–4.5 mIU/L	Close to the standard laboratory upper limit
50–69 years	~4.5–5.5 mIU/L	Mildly raised values are common and often need no treatment
70–79 years	~5.5–6.5 mIU/L	A TSH of 5 or 6 is frequently a normal age-related finding
80+ years	~7.5 mIU/L	Treating to a young adult target may do more harm than good

These figures describe how the population distribution shifts with age; they are not a substitute for the interval your own laboratory prints, which is what your result should be read against. The practical consequence is that in an older person with a mildly raised TSH and a normal free T4, the finding is often watched rather than treated. Applying age-, sex- and race-specific intervals to the NHANES data reclassified 48.5% of people labelled with subclinical hypothyroidism as normal.

Age also affects the lower end differently in pregnancy, where hCG stimulates the thyroid directly and TSH commonly falls below the non-pregnant lower limit in the first trimester. Pregnancy-specific intervals should always be used in that setting.

Subclinical thyroid disease

Subclinical Hypothyroidism: high TSH + normal T4

TSH between 4.0–10 mIU/L with normal free T4 = subclinical hypothyroidism. Many people have no symptoms. Whether to treat depends on TSH level, symptoms, age, and pregnancy status. Pregnant women with subclinical hypothyroidism are always treated: it affects fetal brain development.

Questions to ask your doctor

Should I also check free T3 and free T4?
Should I test thyroid antibodies (anti-TPO) to check for Hashimoto's?
My TSH is 5.5. Do I need medication or just monitoring?
I'm pregnant. What TSH level is safe for my baby?

Why TSH is the key thyroid test

TSH (thyroid-stimulating hormone) comes from the pituitary gland and controls how much hormone the thyroid makes. It works like a thermostat: when thyroid hormone is low, TSH rises to stimulate the gland; when hormone is high, TSH falls. Because this feedback is so sensitive, TSH usually changes before thyroid hormone levels move outside their normal range, making it the best first-line screening test.

A high TSH points to an underactive thyroid, while a low TSH points to an overactive thyroid, with free T4 measured to confirm the picture.

What abnormal TSH means

A raised TSH with low thyroid hormone confirms hypothyroidism, most commonly from the autoimmune condition Hashimoto's thyroiditis. A suppressed TSH with high thyroid hormone confirms hyperthyroidism, often due to Graves' disease. Symptoms guide testing: fatigue, weight gain, cold intolerance and low mood suggest an underactive thyroid, while weight loss, palpitations, anxiety and heat intolerance suggest an overactive one.

Mildly abnormal TSH with normal thyroid hormone ('subclinical' disease) is common and often just monitored, as it may be temporary.

Monitoring treatment with TSH

For people taking levothyroxine for an underactive thyroid, TSH is the main measure used to adjust the dose. Because the thyroid system responds slowly, TSH is usually rechecked around 6–8 weeks after any dose change rather than sooner.

Timing and consistency help: TSH is naturally higher in the early morning, and biotin supplements can interfere with the test, so it is worth mentioning any supplements you take.

Frequently Asked Questions

Do I need to fast for a TSH test?

No fasting is required, but as TSH varies through the day, a consistent time (often morning) is preferred when monitoring treatment.

What is a normal TSH level?

Most laboratories use a range of roughly 0.4–4.0 mU/L, though the ideal target is individualised, particularly in pregnancy and older age.

Can supplements affect my TSH result?

Yes. Biotin (vitamin B7), common in hair and nail supplements, can interfere with some thyroid assays and give misleading results, so pause it before testing if advised.

How soon after changing my thyroid dose should TSH be checked?

Usually after about 6–8 weeks, because the thyroid system takes time to reach a new steady state.

References

The clinical information on this page is based on peer-reviewed sources indexed in PubMed, the biomedical literature database of the US National Library of Medicine.

Koulouri O, Moran C, Halsall D, Chatterjee K, Gurnell M. Pitfalls in the measurement and interpretation of thyroid function tests. Best Pract Res Clin Endocrinol Metab. 2013;27(6):745-62. doi:10.1016/j.beem.2013.10.003 · PMID 24275187
Pearce EN. Management of hypothyroidism and hypothyroxinemia during pregnancy. Endocr Pract. 2022;28(7):711-718. doi:10.1016/j.eprac.2022.05.004 · PMID 35569735
Laurberg P, Andersen S, Carlé A, Karmisholt J, Knudsen N, Pedersen IB. The TSH upper reference limit: where are we at? Nat Rev Endocrinol. 2011;7(4):232-9. doi:10.1038/nrendo.2011.13 · PMID 21301488
Li Q, Tang Y, Yu X, et al. Thyroid function reference intervals by age, sex, and race: a cross-sectional study. Ann Intern Med. 2025;178(7):921-929. doi:10.7326/ANNALS-24-01559 · PMID 40324200
Vadiveloo T, Donnan PT, Murphy MJ, Leese GP. Age- and gender-specific TSH reference intervals in people with no obvious thyroid disease in Tayside, Scotland: the Thyroid Epidemiology, Audit, and Research Study (TEARS). J Clin Endocrinol Metab. 2013;98(3):1147-53. doi:10.1210/jc.2012-3191 · PMID 23345094
Subclinical Hypothyroidism. In: StatPearls. Treasure Island (FL): StatPearls Publishing. NCBI Bookshelf NBK536970