Thyroid Stimulating Hormone (TSH) is a pituitary hormone that regulates the production and release of thyroid hormones from the thyroid gland. This test measures TSH levels in the blood to evaluate thyroid function and is considered the most sensitive indicator of thyroid status. TSH levels are inversely related to thyroid hormone levels - when thyroid hormones are low, TSH rises to stimulate the thyroid; when thyroid hormones are high, TSH falls to reduce stimulation. Elevated TSH typically indicates hypothyroidism (underactive thyroid), while low TSH suggests hyperthyroidism (overactive thyroid) or excessive thyroid hormone replacement. This test is essential for diagnosing thyroid disorders, monitoring thyroid hormone therapy, and screening for thyroid disease in patients with symptoms like fatigue, weight changes, mood disturbances, or heart problems.