Test used to evaluate thyroid function. A high TSH level indicates that the thyroid gland is failing because of a problem that is directly affecting the thyroid known as primary hypothyroidism. A low TSH level indicates that the person has an overactive thyroid that is producing too much thyroid hormone (hyperthyroidism). Occasionally, a low TSH may result from an abnormality in the pituitary gland, which prevents it from making enough TSH to stimulate the thyroid (secondary hypothyroidism). In most healthy individuals, a normal TSH value means that the thyroid is functioning normally. T4 circulates in the blood in two forms: T4 bound to proteins that prevent the T4from entering the various tissues that need thyroid hormone and Free T4, which does enter the various target tissues to exert its effects. The free T4 fraction is the most important to determine how the thyroid is functioning. Individuals who have hyperthyroidism will have an elevated FT4, whereas patients with hypothyroidism will have a low level of FT4.