TSH (Thyroid-stimulating hormone)

Thyroid-stimulating hormone (TSH) is released by your pituitary gland. It controls how much thyroid hormone you make, through negative feedback from the circulating thyroid hormones T3 and T4. In primary thyroid disease, TSH is the single most sensitive marker of your thyroid status. A high value usually means an underactive thyroid (hypothyroidism). A low value suggests an overactive thyroid (hyperthyroidism), or too much hormone from outside. There is one catch. In central hypothyroidism (a pituitary or hypothalamic problem), TSH can look normal or low even though fT4 is low. So you read it together with free T4. Reference ranges also drift slightly higher with age, and mild thyroid trouble is common in older adults.

Last reviewed: June 22, 2026

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