Human Endocrine System and Hormones

Vasopressin (antidiuretic hormone) is produced by the hypothalamus and released by the pituitary. It acts on the kidneys to increase water reabsorption, thereby concentrating urine and conserving body water.

Growth hormone (somatotropin) primarily stimulates the growth of muscle, cartilage, bone and connective tissues. It promotes protein synthesis and cell division in these tissues, making it essential for physical growth.

Thyroxine (T4) is the main hormone secreted by the thyroid gland and controls tissue metabolism by regulating basal metabolic rate. It influences oxygen consumption, heat production, and metabolic activity in cells.

Thyroxine is a metabolic stimulant - it increases tissue metabolism, oxygen consumption, pulmonary ventilation, and basal metabolic rate. "Depression of metabolism" means a decrease in metabolic rate, which is the opposite of what thyroxine does. Therefore, depression of metabolism is NOT caused by thyroxine.

The thyroid gland secretes thyroxine continuously, not in bursts or periodically. This continuous secretion maintains steady metabolic control. Seasonal secretion or puberty-only secretion would cause metabolic fluctuations.

Oxytocin is a small peptide hormone composed of 9 amino acids. It is not a large peptide, fatty acid, or sugar molecule. Peptide hormones are classified by their amino acid chain length.

Goitre (thyroid enlargement) has multiple causes: iodine deficiency prevents thyroxine synthesis, increased TSH production stimulates thyroid growth, and increased thyroxine production can also cause enlargement. All listed mechanisms independently cause goitre.

Growth hormone (somatotropin) is directly secreted by the pituitary gland. Release factors and inhibiting factors come from the hypothalamus, not the pituitary. The pituitary secretes the hormones themselves, not the release factors.

The thyroid gland obtains iodine from the bloodstream, not from lymph, kidney, or liver. Iodine absorbed from diet enters blood circulation, which is then taken up by thyroid follicular cells for hormone synthesis.

Thyroxine is an iodinated hormone - it contains iodine atoms in its molecular structure. This is why iodine is essential for thyroid function. Thyroxine does not inhibit or stimulate iodine compound synthesis in the body.

Thyroxine controls metamorphosis timing in frogs. Excess thyroxine accelerates development, causing earlier metamorphosis. Too little thyroxine delays or prevents metamorphosis. This is a key example of hormonal timing control.

The adrenal gland produces two distinct hormone types: corticosteroids (cortisol, aldosterone) from the cortex and catecholamines (adrenaline) from the medulla. Other glands listed produce only one hormone type.

The thyroid gland changes size based on physiological conditions like puberty, pregnancy, stress, or disease. It does not remain constant. While age and pregnancy can affect size, "different physiological conditions" is the most comprehensive answer.

Adrenal glands regulate blood pressure through aldosterone (cortex) which controls fluid balance, and adrenaline (medulla) which increases heart rate and vascular tone. Thyroid, thymus, and corpus luteum do not directly control blood pressure.

Adrenaline (epinephrine) is produced by the adrenal medulla. Corticosterone, aldosterone, and cortisol are produced by the adrenal cortex, not the medulla. The medulla specializes in catecholamine production.

Epinephrine, thyroxine, and glucagon are all hyperglycemic hormones that increase blood glucose levels. Epinephrine stimulates glycogenolysis, thyroxine increases metabolic rate and gluconeogenesis, and glucagon directly promotes glucose production. Aldosterone is a mineralocorticoid that regulates sodium and potassium balance in the kidneys, but does not affect blood glucose levels directly.

Glucocorticoids (such as cortisol) are catabolic hormones that stimulate proteolysis - the breakdown of proteins into amino acids, particularly during fasting, stress, or hibernation. These amino acids can then be used for gluconeogenesis to maintain blood glucose levels. Secretin stimulates digestive secretions, adrenalin (epinephrine) is primarily involved in acute stress response, and enterogastrone inhibits gastric secretions.

Prolactin is a peptide hormone produced by the anterior pituitary gland that stimulates milk production. Testosterone, oestradiol-17B, and progesterone are all steroid hormones derived from cholesterol, characterized by their four-ring structure. Steroid hormones include sex hormones (androgens, estrogens, progestins) and corticosteroids.

Oestrogens, androgens, progestins, and corticosteroids are all classes of steroid hormones. They share a common chemical structure based on four fused rings and are synthesized from cholesterol. Prostaglandins are lipid-derived eicosanoids, juvenile hormones are insect growth regulators, and these options don't fit the classification.

The parathyroid glands are endocrine glands that secrete parathyroid hormone (PTH), which regulates calcium and phosphate metabolism. PTH increases blood calcium levels by stimulating bone resorption, increasing calcium reabsorption in kidneys, and activating vitamin D. Salivary, sweat, and mammary glands are exocrine glands that secrete substances through ducts to epithelial surfaces.

Glucagon is the primary hormone that increases blood glucose levels during fasting, exercise, or stress. It stimulates glycogenolysis (breakdown of glycogen to glucose) in the liver and gluconeogenesis (formation of glucose from non-carbohydrate sources). Insulin lowers blood glucose by promoting glucose uptake, somatostatin inhibits both glucagon and insulin secretion, so 'all of the above' is incorrect.

Melatonin inhibits estrous (reproductive cycle) in healthy rats, particularly in response to longer dark periods (winter conditions). Melatonin is secreted by the pineal gland in response to darkness and regulates seasonal reproductive cycles in many mammals. It can suppress estrous in light-induced constant estrous rats by maintaining natural photoperiodic signaling.

Androgens (such as testosterone) are primarily secreted by the testes in males. While the adrenal cortex does produce small amounts of androgens, the testes are the main source. The ovaries produce small amounts of androgens as precursors to estrogen, and the parathyroid secretes parathyroid hormone, not androgens. Therefore, the testis is the correct answer as the primary androgen-secreting gland.