The thyroid gland produces hormones that regulate metabolism. Thyroid function tests measure levels of thyroid hormones like TSH, T3, and T4 to determine if the thyroid is functioning properly. Abnormal thyroid function can cause disorders like hypothyroidism (underactive thyroid) or hyperthyroidism (overactive thyroid). Common thyroid disorders include hypothyroidism, hyperthyroidism, goiter, nodules, and cancer.