TSH
What it measures, and what changes the number.
Thyroid stimulating hormone (TSH) is made by the pituitary gland and signals the thyroid to produce thyroxine (T4) and triiodothyronine (T3). The test measures TSH in serum and is the most common first test of thyroid function, because the pituitary adjusts TSH in response to circulating thyroid hormone.
It is usually read with Free T4, often with Free T3, and with thyroid antibodies such as anti-TPO and anti-thyroglobulin when an autoimmune cause is being considered.
TSH follows a daily rhythm, tending to be higher at night and in the early morning, so the time of the draw can shift the value. It is measured by immunoassay, and some assays can be affected by high intake of biotin or by unusual antibodies in the sample. Reports may list it as Thyroid Stimulating Hormone or Hormona estimulante del tiroides (TSH).
Reference ranges labs publish
Each lab states an interval for its own method and the population it measured. They differ, they are not interchangeable, and none of them is a verdict on a single result.
- Everyone, 20 years and over: 0.3 to 4.2 mIU/L · ECLIA (immunoassay) · Source: Mayo Clinic Laboratories, STSH (Thyroid-Stimulating Hormone-Sensitive (s-TSH), Serum) (read 2026-09-24)
- Everyone, 11 years and over: 0.45 to 4.5 μIU/mL · ECLIA (immunoassay) · Source: Labcorp, 004259 (Thyroid-stimulating Hormone (TSH)) (read 2026-09-24)
Often read alongside: Free T4, Free T3, Total T4, Anti-TPO antibodies, Anti-thyroglobulin antibodies