A TSH test is used to check the level of thyroid-stimulating hormone in the blood. Your TSH level can indicate if your thyroid gland is working properly. While a TSH test is usually used as the initial blood test to detect hypothyroidism, the doctor may need to order additional tests to diagnose hypothyroidism. The doctor also may check your level of FT4, or free thyroxine, which is the amount of thyroxine in your body.