The thyroid, and endocrine gland, synthesizes stores and secretes two hormones, thyroxine or tetraiodothyronine (T4), and triiodothyronine (T3), that are chemically related and important to human growth and metabolism. Located below the larynx (voice box), the thyroid's two lobes occurs on either side of the wind-pipe, connected by an isthmus (band of tissue). The gland is composed of many hollow sacs (follicles) filled with colloid (a gelatin material), which contains thyroglobulin, the storage form of the hormones. Essential to the synthesis of the hormones is inorganic iodide, which diffuses from citonin of calcitonin, acts against excessive levels of calcium in the blood and against the effects of parathyroid hormone on bone resorption.
Thyroid hormone secretion is controlled by thyroid stimulating hormone (TSH), or thyrotropin, from the anterior pituitary. In turn the resultant increase in the level of thyroid hormones in the blood serves to signal the pituitary to stop releasing the thyrotropin. This haemostatic mechanism keeps the level of thyroid hormones in the circulatory system within a constant range. A lack of thyroid function in infants causes cretinism, whereas a loss or low levels of the thyroid hormones later in life result in hypothyroidism, or myxedema and possibly Goiter. Over production of the hormones, or hyperthyroidism, also may result in goiter.
Thyroid Function Test
A thyroid function test measures the efficiency of thyroxine and triiodothyronine production by the thyroid gland. These two thyroid hormones which contain iodine, regulate body metabolism. If the hormones are deficient, as in the condition called myxedema, metabolism is slowed down. If they are in excess, as in exophthalmic Goiter, metabolism is accelerated. The oldest method of measuring thyroid function is the determination of the basal metabolic rate, or BMR. The relationship of the BMR to thyroid hormone levels, however, is indirect and inexact. The BMR test has thus been superseded by such test as the radiation method, which measures the rate at which injected radioactive iodine become concentrated in the thyroid gland. This rate is directly related to the rate of thyroid hormone synthesis. Other test include measurement of the competitive protein binding of thyroxine, serum thyroxine, the level of thyroid stimulating hormone (TSH), free thyroxine in blood and urine, and triiodothyronine estimations.
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