Thyroid-Stimulating Hormone (TSH)
Also known as: Thyrotropin, Thyroid function test
TSH is the pituitary gland's signal to the thyroid — it rises when the thyroid is underactive and falls when it is overactive. It is the single most sensitive test for detecting thyroid dysfunction.
Venous blood (serum separator tube)
4–8 hours
Not required
Thyroid Function
Reference Ranges
| Measurement | Reference Range | Unit |
|---|---|---|
| TSH (adults) | 0.4 – 4 | mIU/L |
| TSH (pregnancy, 1st trimester) | 0.1 – 2.5 | mIU/L |
| TSH (pregnancy, 2nd–3rd trimester) | 0.2 – 3 | mIU/L |
| TSH (elderly, >70) | 0.4 – 6 | mIU/L |
Reference ranges are typical adult values. Your laboratory may use slightly different intervals — always use your lab's reported reference.
What This Test Measures
TSH is produced by the pituitary gland and acts as a thermostat — when thyroid hormone levels drop, TSH rises to stimulate more production. When thyroid hormones are too high, TSH is suppressed. This feedback loop makes TSH highly sensitive: even small changes in thyroid hormone output cause large swings in TSH, making it the ideal first-line screening test.
When Is This Test Ordered?
- Screening for hypothyroidism or hyperthyroidism
- Monitoring thyroid replacement therapy (levothyroxine dosing)
- Investigating unexplained fatigue, weight change, or hair loss
- Infertility or irregular periods workup
- Routine check in patients with autoimmune conditions
When Results Are High
High TSH (above 4.0 mIU/L) indicates the pituitary is working hard because the thyroid is underproducing hormones — hypothyroidism. Symptoms: fatigue, cold intolerance, constipation, weight gain, hair loss, slow heart rate.
When Results Are Low
Low TSH (below 0.4 mIU/L) signals suppressed pituitary because thyroid hormones are too high — hyperthyroidism. Symptoms: anxiety, heat intolerance, weight loss, rapid heart rate, tremor. Very low TSH also occurs in over-replaced patients on levothyroxine.
TSH alone vs. the full thyroid panel
| Condition | TSH | Free T4 | Free T3 |
|---|---|---|---|
| Primary hypothyroidism | High ↑ | Low ↓ | Low/normal ↓ |
| Hyperthyroidism | Low ↓ | High ↑ | High ↑ |
| Subclinical hypothyroidism | High ↑ | Normal | Normal |
| Subclinical hyperthyroidism | Low ↓ | Normal | Normal |
| Secondary hypothyroidism | Low/normal | Low ↓ | Low ↓ |
Subclinical thyroid dysfunction — treat or watch?
Subclinical hypothyroidism means TSH is elevated but T4 is still normal. Treatment is generally recommended when TSH exceeds 10 mIU/L or when the patient is pregnant, has symptoms, or has cardiovascular disease. Between 4–10 mIU/L, guidelines vary — the decision is individualised.
Subclinical hyperthyroidism (low TSH, normal T4/T3) carries risk of atrial fibrillation and osteoporosis — especially in older women — and often warrants treatment at TSH below 0.1 mIU/L.
Factors That Can Affect Your Result
- Sample timing matters — TSH peaks between midnight and 8am
- Biotin supplements (above 5 mg/day) can falsely lower TSH in many immunoassays
- Acute non-thyroidal illness temporarily suppresses TSH (sick euthyroid)
- Lithium and amiodarone significantly affect thyroid function
- Pregnancy narrows the reference range significantly — use trimester-specific ranges
Tracking This Test Over Time
For patients on levothyroxine, the target TSH is usually 0.5–2.5 mIU/L. Always measure TSH at the same time of day (ideally morning, before your dose) for valid trend comparisons.
Track your TSH in LoomsRelated Tests
Medical disclaimer
This page is for educational purposes only and does not constitute medical advice. Reference ranges vary between laboratories, and results must be interpreted in the context of your symptoms, history, and other tests. Always discuss your results with your healthcare provider. Content reviewed by the Looms medical team, 2026-06-01.