Thymus

The thymus is a vascular organ of the lymphatic system situated just behind the breastbone. The human thymus continuous to grow for about a year after birth, reaching a weight of about 42 g; this size is maintained until puberty. After puberty the lymphatic tissue is replaced by fat, but the thymus remains functional throughout life.

The main function of thymus is to process lymphocytes received from the blood producing bone marrow and fetal liver. These cells proliferate and differentiate in the thymus into thymic lymphoid cells called T cells, each one programmed for the number of antigens to which it will react. In humans the cellular immune system requires the presence of the thymus at birth; this system allows the body to recognize foreign, that is “non self” tissue and to attack malignant cells, viral infection, fungal infection, and some bacteria. Little is known of the factors and processes of thymic function. The importance of the thymus to the human immune system, however, is readily demonstrated in some patients with congenital thymic deficiency states by the restoration of immunological responsiveness after fetal thymus graft.

The thymus gland is an organ of the lymphatic system, which protect the body against infection. Located behind the sternum, near the heath and lungs, it is well supplied with blood vessel. Its two main lobes are each subdivided into numerous lobules; a network of delicate connective tissue holds the lobes together. Within each lobule are two zones of tissue, inner zones called the cortex and an outer zone called the medulla. The cortex is composed of lymphocytes, while blood cells that produced antibodies and attack bacteria; this lymphocytes are packed into a fiber structure called a reticulum. The medulla has a more cellular reticulum and contains thymic corpuscles, which are concentric clusters of epithelial cells enclosing a core of granular cells. The function of these corpuscles is not yet understood. The thymus is most active during fetal and childhood growth. Its main function appearing to be the production of lymphocytes and the destruction of defective lymphocytes. The thymus may also secrete a hormone that influence the response of lymphocytes for foreign tissue. After puberty the thymus slowly degenerates.

Thyroid Gland

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.