Thyroxine (T4) and Triiodothyronine (T3)
Synthesized in the thyroid gland. They stimulate oxygen consumption, increase body heat, metabolism and heart rate. There is an increase in the dissociation of oxygen from hemoglobin by increasing red cell 2,3-diphosphoglycerate (2,3-DPG)
and an increase in the activity of the Na+-K+ ATPase in many tissues thus increasing energy consumption.
Mitochondrial protein synthesis and carbohydrate absorption from the GI tract are enhanced.
It increases the number and affinity of beta-adrenergic receptors in the heart, thus increasing the cardiac sensitivity to catacholamines. It also potentiates the effect of GH on tissue.
Parathyroid Hormone (PTH)
Synthesized and released from chief cells in the parathyroid gland. PTH promotes increase in serum calcium by increased release from bones, decreased renal excretion, and enhanced absorption from the intestine via vitamin D3. It stimulates osteoclasts and inhibits osteoblasts, thus increasing resorption of calcium and phosphate from bone and inhibits phosphate reabsorption in the renal tubule, thus increasing phosphate excretion in the urine. PTH stimulates hydroxylation of 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D in the kidney and renal secretion of bicarbonate.
Calcitonin is secreted from the parafollicular cells or clear cells (ultimobranchial bodies) in the thyroid gland and inhibits bone resorption and produces hypocalce-mia in experimental animals. Its secretion results in decreases in serum phosphate levels but it has not been demonstrated to be important in the basal control of serum calcium during normal homeostasis in humans. Salmon calcitonin is more than 20 times more active in humans than human calcitonin.
Produced and secreted from the adrenal cortex and stimulates release of gluca-gons. It increases blood glucose by increasing gluconeogenesis in the liver and decreasing insulin-stimulated glucose uptake in peripheral tissues and decreases peripheral amino acid uptake and protein synthesis but increases hepatic uptake of amino acids. It also increases protein catabolism and peripheral lypolysis.
Immunologic effects include a decrease in interleukin-2 production, inhibition of B-cell activation and proliferation, inhibition of monocyte and neutrophil migration to areas of inflammation and decreased histamine release and histamine-induced lysosomal degranulation by mast cells.
Stimulates angiotensinogen release and decreases the release of the vasodilator prostaglandin I2. Cortisol inhibits collagen formation, fibroblast activity, and formation of bone by osteoblasts with a decrease plasma calcium levels by inhibiting osteoclast formation and activity. It provides feedback inhibition of ACTH and CRH release in the anterior pituitary and hypothalamus, respectively
Aldosterone is secreted from the zona glomerulosa of the adrenal gland in response to hyperkalemia, circulating angiotensin II, surgery, physical trauma, hemorrhage, or anxiety. It stimulates sodium retention and potassium and hydrogen ion secretion in the distal convoluted tubule of the kidney and promotes sodium absorption by the intestinal mucosa, sweat glands, and salivary glands.
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