Hormones Health Article

Thyroid gland

The thyroid lies under the larynx and synthesizes two hormones, thyroxine and tri-iodothyronine. This gland takes up iodine from the blood and has the highest iodine level in the body. The iodine is incorporated into the thyroid hormones. Thyroxine has four iodine atoms and is called T4. Tri-iodothyronine has three iodine atoms and is called T3. Both T3 and T4 function to increase the metabolic rate of several cells and tissues. The brain, testes, lungs, and spleen are not affected by thyroid hormones, however. T3 and T4 indirectly increase blood glucose levels as well as the insulin-promoted uptake of glucose by fat cells. Their release is modulated by TRHRH from the hypothalamus. When temperature drops, a metabolic increase is triggered by TSH. Chronic stress seems to reduce TSH secretion which, in turn, decreases T3 and T4 output.

Depressed T3 and T4 production is the trademark of hypothyroidism. If it occurs in young children, this decreased activity can cause physical and mental retardation. In adults, it creates sluggishness—mentally and physically—and is characterized further by weight gain, poor hair growth, and a swollen neck. Excessive T3 and T4 cause sweating, nervousness, weight loss, and fatigue. The thyroid also secretes calcitonin, which serves to reduce blood calcium levels. Calcitonin's role is particularly significant in children whose bones are still forming.

Parathyroid glands

The parathyroid glands are attached to the bottom of the thyroid gland. They secrete the polypeptide parathyroid hormone (PTH), which plays a crucial role in monitoring blood calcium and phosphate levels. Calcium is a critical element for the human body. Even though the majority of calcium is in bone, it is also used by muscles, including cardiac muscle, for contractions, and by nerves in the release of neurotransmitters. Calcium is a powerful messenger in the immune response of inflammation and blood clotting. Both PTH and calcitonin regulate calcium levels in the kidneys, the gut, bone, and blood.

PTH deficiency can be due to autoimmune diseases or to inherited parathyroid gland problems. Low PTH capabilities cause depressed blood calcium levels and neuromuscular problems. Very low PTH can lead to tetany or muscle spasms. Excess PTH can lead to weakened bones because it causes too much calcium to be drawn from the bones and to be excreted in the urine. Abnormalities of bone mineral deposits can lead to a number of conditions, including osteoporosis and rickets. Osteoporosis can be due to dietary insufficiencies of calcium, phosphate, or vitamin C. The end result is a loss of bone mass. Rickets is usually caused by a vitamin D deficiency and results in lower rates of bone formation in children. These examples show the importance of a balanced, nutritious diet for healthy development.

Adrenal glands

The two adrenal glands sit one on top of each kidney. Both adrenals have two distinct regions. The outer region (the medulla) produces adrenaline and noradrenaline and is under the control of the sympathetic nervous system. The inner region (the cortex) produces a number of steroid hormones. The cortical steroid hormones are derived from cholesterol and include mineralocorticoids (mainly aldosterone), glucocorticoids (mainly cortisol), and gonadocorticoids. Aldosterone and cortisol are the major human steroids in the cortex. However, testosterone and estrogen are secreted by adults (both male and female) at very low levels.

Aldosterone plays an important role in regulating body fluids. It increases blood levels of sodium and water and lowers blood potassium levels. Cortisol secretion is stimulated by physical trauma, exposure to cold temperatures, burns, heavy exercise, and anxiety. Cortisol targets the liver, skeletal muscle, and adipose tissue, and its overall effect is to provide amino acids and glucose to meet synthesis and energy requirements for metabolism and during periods of stress. Because of its anti-inflammatory action, cortisol is used clinically to reduce swelling. Excessive cortisol secretion leads to Cushing's syndrome, which is characterized by weak bones, obesity, and a tendency to bruise. Cortisol deficiency can lead to Addison's disease, which has the symptoms of fatigue, low blood sodium levels, low blood pressure, and excess skin pigmentation.

The adrenal medullary hormones are epinephrine (adrenaline) and nor-epinephrine (nor-adrenaline). Both of these hormones serve to supplement and prolong the "fight or flight" response initiated in the nervous system. This response includes increased heart rate, peripheral blood vessel constriction, sweating, spleen contraction, glycogen conversion to glucose, dilation of bronchial tubes, decreased digestive activity, and low urine output.