Thyroid function tests are blood tests which help to check the function of the thyroid gland. They are mainly used to detect hypothyroidism (underactive thyroid) and hyperthyroidism (overactive thyroid).Note: the information below is a general guide only.hese tests may help doctors to find a specific cause of the low TSH. Thyroid function tests are usually done to find out whether the thyroid gland is working properly. This is mainly to diagnose underactive thyroid (hypothyroidism) and overactive thyroid (hyperthyroidism).