TSH (Thyroid Stimulating Hormone) Test

TSH, or Thyroid Stimulating Hormone, is produced by the pituitary gland and acts as the master signal that tells your thyroid gland how much hormone to produce.

Because of this feedback relationship, TSH is typically the single most sensitive marker of thyroid function — even before actual thyroid hormone levels (T3 and T4) shift outside their normal range, TSH often changes first, making it the preferred first-line screening test for thyroid disorders.

When the thyroid is underactive, the pituitary responds by producing more TSH to try to stimulate it, so TSH rises. When the thyroid is overactive, the pituitary senses excess hormone and reduces TSH production.

This inverse relationship is why doctors almost always start a thyroid workup with TSH before deciding whether additional tests like Free T3 and Free T4 are needed.