 Dear students, in this topic we shall discuss thyroid gland and its hormones. Thyroid gland lies on the ventral surface of trachea just below the Adam's apple. It consists of two lobes, right and left lobes. There are two types of glandular cells in a thyroid gland. These are follicular cells which secrete two hormones, thyroxine, which is also known as triiodothyronine or T4 and triiodothyronine that is T3. The second type of cells in thyroid gland are para follicular cells which produce one hormone which is called calcitonine. The secretion of T3 and T4, we shall discuss the secretion of T3 and T4. The secretion of T4 dominates T3, however T3 is more active form and carries the major metabolic functions. So the target cells mostly convert T4 into T3 which is active form. We shall discuss the secretion of T3 and T4 in detail. The release of thyroid hormone is stimulated by the thyroid stimulating hormone from anterior pitch of T. This TSH is released on the stimulus of TRH from hypothalamus. This is a positive feedback loop. The stimulus for this positive feedback are stress, cold and low skin temperature and low metabolic rate. In these conditions, TRH from hypothalamus is secreted which stimulates TSH release and TSH stimulates the release of thyroid hormones. The release of T3 and T4 that is thyroid hormones puts a negative feedback to the hypothalamus and pituitary. As a result, the TRH and TSH production is reduced which affect the production of thyroid hormones. In this way, the negative feedback controls the quantity of thyroid hormones. Now we shall discuss the functions of T3 and T4. These hormones have diverse effects on the physiology of virtually all tissues of the body. Their roles can be grouped into two categories, metabolic roles and developmental roles. First we shall discuss the metabolic roles of T3 and T4 that is thyroid hormones. They stimulate cellular respiration, oxygen consumption and metabolic rate. And as an effect of all these three processes, the heat production. These heat production through this thyroid action, thyroid hormones action is very important in thermoregulation. These hormones also sensitize some tissues to epinephrine which is a hormone of adenyl medulla. We are discussing the roles of T3 and T4 which also promote the normal motility of gastrointestinal track. They also regulate some reproductive functions. Now we shall discuss the developmental roles of thyroid hormones. These hormones significantly affect development and maturation of vertebrate animals. They are involved in the normal functioning of bone-forming cells and the branching of nerve fibres during embryonic development of brain. They also control metamorphosis of tadpole larvae into adult. The developmental effect of growth hormone occurs only in the presence of thyroid hormones. Dear students, Thyroid gland thyroid hormones is a disease called Graves disease. It is an autoimmune disease which is because of production of such antibodies which mimic TSH. They bind to the receptors for TSH and cause sustained production of thyroxine. As a result, the symptoms of Graves disease are produced which include high body temperature, very high level of sweating, weight loss, irritability and high blood pressure and the most common symptoms of Graves disease is the protruding eyes or exothermia. Dear students, Hypothyroidism that is the production of thyroid hormones in low quantity results in two diseases in men. These include the creatinism and goiter. Creatinism results from iodine deficiency in early stages of development. In creatinism, the somatic, neural and sexual development of a person is severely affected. Moreover, metabolic rate is reduced and resistance to disease is also reduced. The inadequate production of thyroxine in adults leads to goiter. In goiter, TSH from pituitary is produced in high quantity that causes excessive stimulation of thyroid glands which become enlarged, that is, in the hypertrophy or is disease goiter. Here we can see the symptoms of goiter in a patient with enlarged throat. This is the symptom of goiter. Inside it we can see the hypertrophy of the thyroid gland. Dear students, both the symptoms of hypothyroidism that is creatinism and goiter can be controlled and treated by adding iodine in diet at the iodine enhances thyroid hormone production. Dear students, now we shall discuss the second hormone of thyroid gland that is the calcitonin. Calcitonin is a protein hormone which is secreted by the para follicular cells of thyroid. It is released in response to hypercalcemia, that is, high level of blood calcium. It is important hormone for calcium metabolism and calcium homeostasis. Calcitonin promotes calcium deposition in bone matrix. It suppresses calcium ions loss from bones. It also enhances calcium ion excretion from the kidneys.