The Thyroid-Stimulating Hormone (TSH) test measures the level of TSH in your blood to evaluate thyroid function. It helps diagnose conditions like hypothyroidism (underactive thyroid) or hyperthyroidism (overactive thyroid) by assessing how well the thyroid gland responds to signals from the pituitary gland. The test is simple, requiring a blood sample, and is often used alongside other thyroid tests like T3 and T4.