A thyroid blood test examines the quantity of thyroid hormones in the body, according to MedicineNet. This test helps determine the status of the hormone production by the thyroid gland. The production status can either be normal, underactive or overactive.
The two major hormones produced by the thyroid glands are thyroxine and triiodothyronine, notes MedicineNet. These hormones constitute 99.9 percent of the blood thyroid hormones, and they help in cell metabolism. If their production process is interfered with, it results in either hypothyroidism or hyperthyroidism, which is thyroid hormone deficiency or excess of it respectively. To diagnose these conditions, a thyroid blood test is necessary.