A TSH test is done to test the thyroid hormone levels. This aims at finding the working pattern of the thyroid gland, and it can help you arrive at a conclusion if the thyroid levels are overactive or underactive. An overactive thyroid is called hyperthyroidism, and an underactive thyroid is called hypothyroidism. When you go for this test, you will be fortunate to know the problems with the thyroid gland, even before the symptoms start appearing. Lots of health problems are associated with the issues with the thyroid gland. A thyroid blood test can rule out them even at the outset.