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Thyroid-stimulating hormone (TSH)

Thyroid · also called Thyrotropin

What it measures

TSH is the pituitary hormone that drives thyroid hormone production. Because it responds sensitively to small changes in thyroid hormone, it is the first-line test for thyroid dysfunction and for adjusting levothyroxine dose.

Specimen and preparation

Specimen: Serum or lithium-heparin plasma
Tube: Gold top
Preparation: Stop high-dose biotin for at least 2 days before sampling (assay interference).

Reference range

Group	Range
Adult	0.4–4.0 mIU/L
Pregnancy, 1st trimester	≈0.1–4.0 (use trimester-specific range) mIU/L
Newborn (day 3–5)	Up to 10–15 mIU/L

Reference ranges differ between laboratories, methods and populations. Always use the range printed on your own lab report.

Causes of high values

Primary hypothyroidism (Hashimoto thyroiditis)
Under-replacement with levothyroxine
Recovery phase of non-thyroidal illness
TSH-secreting pituitary adenoma (rare)
Amiodarone or lithium

Causes of low values

Hyperthyroidism (Graves disease, toxic nodules)
Over-replacement with levothyroxine
Central hypothyroidism
Early pregnancy
Steroids and dopamine

Method

Third-generation chemiluminescent immunoassay

Good to know

Always interpret with free T4 when TSH is abnormal. Allow 6–8 weeks after dose changes before rechecking.

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