TSH (thyroid-stimulating hormone)
Also known as: TSH, thyrotropin, thyroid stimulating hormone
The pituitary hormone that drives the thyroid gland. Because the pituitary responds to tiny changes in thyroid hormone, TSH moves in the opposite direction to thyroid function and is the single most sensitive test of thyroid status.
- Also known as
- TSH, thyrotropin, thyroid stimulating hormone
- Units
- mU/L (mIU/L)
- Measured by
- TSH test, Thyroid function tests
What it is and what it does
The hypothalamus releases TRH, the pituitary responds with TSH, and TSH tells the thyroid to make thyroxine (T4) and triiodothyronine (T3); T4 and T3 then feed back to switch TSH off. When the thyroid fails, T4 falls and TSH climbs, often to many times the upper limit; when the thyroid is overactive, TSH is suppressed towards zero. The relationship is logarithmic, so TSH changes a great deal for a small change in T4, which is why it is the screening test and the value used to adjust levothyroxine. It takes about six weeks to settle after a dose change. TSH is unreliable when the pituitary itself is diseased, in the recovery phase of severe illness, and in the first trimester of pregnancy, when the placental hormone hCG mimics it.
Why it may be higher
- Primary hypothyroidism: autoimmune (Hashimoto) thyroiditis, after thyroid surgery or radioiodine, iodine deficiency
- Under-replacement with levothyroxine, or poor absorption of it (taken with food, iron, calcium, PPIs)
- Subclinical hypothyroidism (raised TSH with normal free T4)
- Recovery from severe illness, some medicines (amiodarone, lithium), rarely a TSH-secreting pituitary tumour
Why it may be lower
- Hyperthyroidism: Graves disease, toxic nodules, thyroiditis
- Over-replacement with levothyroxine
- Pituitary or hypothalamic disease (secondary hypothyroidism: low TSH with low T4)
- First trimester of pregnancy, severe non-thyroidal illness, high-dose steroids and dopamine
A high or low value can be associated with several different situations. The result is interpreted with other results, symptoms, history and clinical findings.
What can affect the result
- Time of day (highest overnight), acute illness, pregnancy
- Biotin supplements interfere with some assays
- Levothyroxine dose timing and adherence
- Age: the reference interval rises in older adults
Medicines that can change it
Each medicine page lists its effect on this marker under "Effects on tests and results", with the source.