What laboratory test is most commonly used to evaluate thyroid function?

The thyroid-stimulating hormone (TSH) test is the most commonly used laboratory test to evaluate thyroid function because it serves as a primary indicator of how well the thyroid gland is functioning. TSH is produced by the pituitary gland and regulates the production of thyroid hormones—namely, T3 (triiodothyronine) and T4 (thyroxine)—by the thyroid gland.

When thyroid hormone levels are low, the pituitary gland releases more TSH to stimulate the thyroid to produce more hormones. Conversely, if thyroid hormone levels are high, TSH production is reduced. Therefore, measuring TSH levels is crucial for assessing whether the thyroid is underactive (hypothyroidism) or overactive (hyperthyroidism).

While the free T4 and total T3 tests provide additional information about thyroid hormone levels, TSH testing is often considered the most sensitive and reliable initial screening test. It helps guide further testing and management decisions. The thyroglobulin test, while useful in certain contexts, is not a routine assessment of thyroid function but rather a tumor marker used in specific cases, like monitoring certain types of thyroid cancer.