The thyroid gland is located just inferior to the larynx. It consists of two lobes, each one lateral to the trachea, that are connected by an anterior isthmus.
Hormones of the Thyroid Gland
|Thyroxine (T4) and triiodothyronine (T3)||TSH from anterior lobe of the pituitary gland||Increase metabolic rate; accelerate growth; stimulate neural activity||Hyposecretion in infants and children causes cretinism; in adults, it causes myxedema. Hypersecretion causes Graves disease. Iodine deficiency causes simple goiter.|
|Calcitonin (CT)||Blood Ca2+ level||Decreases blood Ca2+ levels by promoting Ca2+ deposition in bones, inhibiting removal of Ca2+ from bones, promoting excretion of Ca2+ by kidneys|
Iodine atoms are essential for the formation and functioning of two similar thyroid hormones, produced by groups of cells forming thyroid follicles that respond to TSH. Thyroxine is the primary hormone. It is also known as T4 because each molecule contains four iodine atoms. The other hormone, triiodothyronine (tri”i-o”do-thi ‘ro-nen) or T3f contains three iodine atoms in each molecule. Both T4 and T3 exert their effect on body cells, and they have similar functions. They increase the metabolic rate, promote protein synthesis, and enhance neuron function. T3 and T4 are the primary factors that determine the basal metabolic rate (BMR), the number of calories required at rest to maintain life. Thyroid hormones are also important during infancy and childhood for normal development of the nervous, skeletal, and muscular systems. Secretion of these hormones is stimulated by TSH from the anterior lobe of the pituitary gland, and TSH, in turn, is regulated by a negative-feedback mechanism as described in the discussion of the anterior lobe of the pituitary gland.
Disorders Hypersecretion, hyposecretion, and iodine deficiencies are involved in the thyroid disorders: Graves disease, simple goiter, cretinism, and myxedema.
Graves disease results from the hypersecretion of thyroid hormones. It is thought to be an autoimmune disorder in which antibodies bind to TSH receptors, stimulating excessive hormone production. It is characterized by restlessness and increased metabolic rate with possible weight loss. Usually, the thyroid gland is somewhat enlarged, which is called a goiter (goy-ter), and eyes bulge due to the swelling of tissues posterior to the eyes, producing what is called an fxophthalmic (ek-sof-thal-mik) goiter.
Simple goiter is an enlargement of the thyroid gland that results from a deficiency of iodine in the diet. Without adequate iodine, hyposecretion of thyroid hormones occurs and the thyroid gland enlarges due to overstimulation with TSH in an attempt to produce more thyroid hormones. In some cases, the thyroid gland may become the size of an orange. Goiter can be prevented by including very small amounts of iodine in the diet. For this reason, salt manufacturers produce “iodized salt,” which contains sufficient iodine to prevent simple goiter.
Cretinism (kre’-tin-izm) is caused by a severe hypo- secretion of thyroid hormones in infants. Without treatment, it produces severe mental and physical retardation. Cretinism is characterized by stunted growth, abnormal bone formation, mental retardation, sluggishness, and goiter.
Myxedema (mik-se-de’-mah) is caused by severe hyposecretion of thyroid hormones in adults. It is characterized by sluggishness, weight gain, weakness, dry skin, goiter, and puffiness of the face.
The thyroid gland produces a third hormone, calcitonin (kal-si-to ‘-nin) (CT), from cells called C cells that are located between thyroid follicles. C cells do not respond to the hormonal mechanism the same as thyroid follicles do but respond to a humoral negative-feedback mechanism linked to blood Ca2 + levels. Calcitonin decreases blood Ca2+ by inhibiting the bone-resorbing action of osteoclasts, increasing the rate of Ca’ + deposition by osteoblasts, and promoting Ca2+ excretion by the kidneys. An excess of Ca2+ in the blood stimulates the thyroid gland to secrete calcitonin. The concentration of Ca’+ in the blood is important because it plays vital roles in metabolism, including maintenance of healthy bones, conduction of nerve impulses, muscle contraction, and clotting of blood. The function of calcitonin is antagonistic to parathyroid hormone.