TSH (Thyroid Stimulating Hormone)
Definition
TSH is a hormone produced by the pituitary gland that tells the thyroid gland how much thyroid hormone it needs to produce. The test measures the level of this hormone in the blood.
Purpose
Evaluates thyroid function and detects hormone imbalances. Used to distinguish between hyperthyroidism (too much hormone) and hypothyroidism (too little hormone). In patients taking thyroid medication it allows dose monitoring, and it provides follow-up after treatment for hyperthyroidism.
Results
| TSH | Usually indicates |
|---|---|
| High | Thyroid hormone is low, so the pituitary is working harder → hypothyroidism |
| Low | Thyroid hormone is high, so the pituitary has reduced its output → hyperthyroidism |
Normal Values
Laboratory-specific. For comparison: the 2017 American Thyroid Association (ATA) guideline recommends an upper limit of 4.0 mIU/L for the first trimester of pregnancy. Non-pregnant adult ranges vary by laboratory.
Risks and Limitations
The test cannot explain why TSH is high or low; additional testing is needed to find the cause. Some serious illnesses unrelated to the thyroid can lower TSH. In people over 80 it may be elevated even without thyroid disease. Pregnancy affects TSH levels and can sometimes give misleading results. It has also been shown that even mild anemia can affect thyroid function tests in pregnancy.