TSH
Also reported as: Thyroid stimulating hormone · Thyrotropin
The pituitary's signal telling the thyroid to produce more hormone. It moves inversely to thyroid output, so a high TSH means an underactive thyroid.
The upper bound is contested; several bodies argue for 2.5–3.0 mIU/L once people with thyroid autoimmunity are excluded.
Most healthy people without thyroid antibodies cluster between 1 and 2 mIU/L, and symptom burden tends to increase above that even within the reference range.
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Where the thresholds sit
| Band | Range (mIU/L) | Note |
|---|---|---|
| Low | < 0.45 | Suggests overactivity — needs assessment. |
| Optimal | 1–2 | — |
| Acceptable | 2–2.5 | — |
| Borderline | 2.5–4.5 | Check thyroid antibodies and free T4. |
| Elevated | > 4.5 | Subclinical or overt hypothyroidism. |
Why it matters
Thyroid function sets basal metabolic rate, and both under- and over-activity affect cardiovascular risk, cognition, mood and body composition. Because the pituitary amplifies small changes in thyroid hormone, TSH is a sensitive early detector — it moves before free T4 leaves its range.
What raises it
- Hashimoto's thyroiditis, the commonest cause
- Iodine deficiency or excess
- Selenium deficiency
- Recovery from acute illness
- Lithium and amiodarone
What lowers it
- Graves' disease
- Excess thyroid hormone replacement
- Some supplements containing iodine or thyroid extract
How to move it
- 1Test in the morning and fasted — TSH follows a diurnal rhythm and can vary by 25% across the day.
- 2If TSH is above 2.5, ask for thyroid peroxidase antibodies and free T4. TSH alone does not tell you why.
- 3Biotin supplements interfere with thyroid immunoassays. Stop them 48 hours before testing or the result may be meaningless.
- 4Ensure adequate iodine and selenium, without megadosing either.
What moves this marker
Interventions on this site with evidence behind them for this specific marker. None of them replaces working out why yours is where it is.
What makes a single reading misleading
- Biotin at supplement doses causes falsely low TSH and falsely high thyroid hormone readings — a common and avoidable source of misdiagnosis.
- TSH is unreliable during acute illness and in the months after.
- The reference range widens with age; a mildly raised TSH in an 80-year-old may need no treatment.
Sources
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Educational only. Reference ranges are not diagnostic thresholds and this page cannot account for your medical history, medication or symptoms. Discuss any result that concerns you with a qualified clinician. See our medical disclaimer.