TSH Test
The TSH (thyroid-stimulating hormone) test is the most important single test for thyroid function. It measures the signal the pituitary sends to the thyroid — and because the pituitary responds sensitively to hormone levels, TSH often changes before thyroid hormones do.
It is a simple blood test requiring no special preparation in most cases.
Last medically reviewed: 1 November 2025
Key points
- TSH is the first-line thyroid screening test.
- High TSH usually indicates an underactive thyroid; low TSH an overactive one.
- Reference ranges vary between laboratories.
- TSH can be misleading in pituitary disease and acute illness.
What TSH measures
TSH is produced by the pituitary gland and stimulates the thyroid to release T4 and T3. When thyroid hormone is low, TSH rises; when hormone is high, TSH falls. This inverse relationship makes TSH an excellent screening marker.
Typical reference ranges are roughly 0.4–4.0 mIU/L, though ranges vary slightly between laboratories.
Interpreting results
High TSH usually indicates hypothyroidism — the pituitary is pushing harder to stimulate an underactive gland. Low TSH suggests hyperthyroidism or over-replacement with thyroid medication.
A normal TSH generally means thyroid function is adequate, though early or mild disease, central (pituitary) problems, or pregnancy can occasionally require additional tests.
Preparation and limitations
No fasting is usually required. Biotin supplements can interfere with some assays — tell your doctor about supplements. If you take levothyroxine, timing matters: TSH is usually checked consistently, often before the morning dose, but follow your clinician’s instructions.
A single abnormal TSH is usually confirmed with a repeat test before diagnosis, since TSH can fluctuate transiently.
When more testing is needed
If TSH is abnormal, free T4 (and sometimes free T3) are checked to determine severity. Anti-TPO antibodies identify autoimmune causes. TRAb testing helps diagnose Graves’ disease. Thyroid ultrasound evaluates structure.
During pregnancy, trimester-specific TSH ranges apply and testing is more nuanced.
Frequently asked questions
Do I need to fast for a TSH test?
No — TSH can be measured at any time of day without fasting in most cases.
Can I take my levothyroxine before the test?
Follow your doctor’s instructions — many clinicians prefer TSH checked before the morning dose for consistency, but this varies.
Is one abnormal TSH enough for diagnosis?
Usually not — a repeat test confirms, since TSH can be transiently affected by illness, medications, or recent changes.
References & further reading
This page is based on established medical knowledge and guidelines from authoritative sources. Always consult a qualified clinician for personal medical advice.
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