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Thyroid
Thyroid-stimulating hormone (TSH)
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A pituitary glycoprotein hormone that stimulates thyroid hormone synthesis under negative feedback from free T4 and T3. It is the first-line screening test for primary thyroid dysfunction.
Reference interval
| Conventional (mIU/L) | SI (mIU/L) | |
|---|---|---|
| Adults | 0.4–4 mIU/L | 0.4–4 mIU/L |
Specimen: Serum · Conventional × 1 = SI
Causes of high values
Primary hypothyroidism (including Hashimoto thyroiditis), subclinical hypothyroidism, recovery phase of non-thyroidal illness, inadequate thyroid hormone replacement, rarely TSH-secreting adenoma or thyroid hormone resistance.
Causes of low values
Hyperthyroidism (Graves disease, toxic nodular goitre, thyroiditis), subclinical hyperthyroidism, excess levothyroxine, central hypothyroidism, first-trimester pregnancy, non-thyroidal illness, glucocorticoids and dopamine.
Pre-analytical and method notes
The range of about 0.4 to 4.0 mIU/L is assay-dependent, with narrower targets used in pregnancy by trimester and in some older adults. Biotin supplements can interfere with immunoassays, and a single abnormal value should be repeated with free T4 before acting. Reference intervals vary by laboratory, method and population; use the interval printed on the report.
References
- Williams Textbook of Endocrinology. Elsevier, 14th edition (2019) — Tier 3 (textbook)
- Rifai N, Horvath AR, Wittwer CT. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. Elsevier, 6th edition (2018) — Tier 3 (textbook)
- Loscalzo J, et al.. Harrison's Principles of Internal Medicine. McGraw-Hill, 21st edition (2022) — Tier 3 (textbook)
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