A thyroid function test may look for free triiodothyronine, free tetraiodothyronine and thyroid stimulating hormone, or it may look for total triiodothyronine, total tetraiodothyronine and thyroid stimulating hormone. It is mainly used to determine if thyroid function is normal. Triiodothyronine and tetraiodothyronine are produced by thyroid follicular cells, with tetraiodothyronine being the predominant one, and are able to enter tissue cells and exert physiological effects. Thyrotropin is produced by the pituitary gland and promotes the secretion of thyroid hormones. When hypothyroidism occurs, triiodothyronine and tetraiodothyronine decrease and thyrotropin increases; when hyperthyroidism occurs, triiodothyronine and tetraiodothyronine increase and thyrotropin decreases. It should be noted that pregnant women or women taking birth control pills have higher levels of thyroid-binding globulin and therefore higher levels of total triiodothyronine and total tetraiodothyronine, which do not accurately reflect the level of thyroid hormones, and should be tested for free triiodothyronine and tetraiodothyronine. If the test results are abnormal, you should consult a doctor promptly.