Thyroid-stimulating hormone (TSH) is produced by the anterior pituitary gland and acts as the primary regulator of thyroid function. When thyroid hormone levels in the blood fall, the pituitary responds by releasing more TSH to stimulate the thyroid to produce more T4 and T3. When thyroid hormones are abundant, TSH is suppressed.

TSH is the most sensitive indicator of thyroid dysfunction and is always the first test ordered when thyroid disease is suspected. An elevated TSH indicates the thyroid is underactive (hypothyroidism), while a suppressed TSH indicates it is overactive (hyperthyroidism). However, TSH alone does not tell the full story — normal TSH does not rule out poor T3 conversion, thyroid antibody activity, or tissue-level thyroid hormone resistance.