Thyroid functioning
The human thyroid gland is at the base of the neck in front of the trachea, or windpipe. It is main hormones, thyroxine and triiodothyronine, have widespread effects on most cells. They are critical for normal development of many tissues, and they control overall metabolic rates in humans and other warm-blooded animals. Their importance is brought into sharp focus by cases of abnormal thyroid secretion.
The synthesis of thyroid hormones requires iodine, which is obtained from the diet. In the absence of iodine, blood levels of these hormones decrease. The anterior pituitary responds to this by secreting a thyroid stimulating hormone (TSH). Because the thyroid hormones cannot be synthesized, the feedback signal continues and so does TSH secretion. Excess TSH overstimulates the thyroid gland and causes it to enlarge. The enlargement is a form of goiter. Goiter caused by iodine deficiency is no longer common in countries where people use iodized salt.
Hypothyroidism results from insufficient concentrations of thyroid hormones. Hypothyroid adults often are overweight, sluggish, dry-skinned, intolerant of cold, and sometimes confused and depressed. Affected women often have menstrual disturbances. Hyperthyroidism results from excess concentrations of thyroid hormones. Hyperthyroid adults show an increased heart, rate, elevated blood pressure, weight loss despite normal caloric intake, intolerance of heat, and profuse sweating. Typically they are nervous, agitated, and have trouble sleeping.
Parathyroid functioning
Some endocrine glands do not respond directly to other hormones or nerves. They respond homeostatically to chemical change in the immediate surroundings. The parathyroid glands are like this. Four such glands are positioned next to the back of the human thyroid. They secrete parathyroid hormone (PTH) when the concentration of calcium ions in their surroundings decreases. Their action affects how much calcium is available for enzyme activation, muscle contraction, blood clotting, and many other tasks.
PTH stimulates bone cells to release calcium and phosphate ions, and stimulates the kidneys to conserve them. PTH also helps activate vitamin D. The activated form, a hormone, enhances calcium absorption from ingested food. In vitamin D deficiency, too little calcium and phosphorus is absorbed, so bones develop improperly. This ailment is called rickets.
Hormonal responses to the external environment influence growth, development, and reproduction in predictable ways. The pineal gland, a photosensitive organ in the brain, provides a good example of this. In the absence of light, the pineal gland secretes the hormone melatonin. Thus bood levels of melatonin vary from day to night, and they vary with the changing seasons. The variations influence the development of primary reproductive cycles and reproductive behavior in a variety of species.
Consider the hamster. In winter, when nights are longest, melatonin levels are high and sexual activity is suppressed. In summer, when days are longer, melatonin levels are low-and hamster sex peaks. Or consider a male white-throated sparrow. Melatonin suppresses growth of the bird’s gonads until spring, when days start to lengthen. Then, stepped-up gonadal activity leads to the production of hormones that influence singing behavior. With his distinctive song, the sparrow defines his territory and may hold the interest of a mate.
Does melatonin also influence human behavior? Perhaps, it seems possible that decreased melatonin secretion triggers puberty, the age at which human reproductive organs and structures start to mature. In cases where disease has destroyed the pineal gland, puberty begins prematurely, the focus essay mentions some other interesting possibilities.
The human thyroid gland is at the base of the neck in front of the trachea, or windpipe. It is main hormones, thyroxine and triiodothyronine, have widespread effects on most cells. They are critical for normal development of many tissues, and they control overall metabolic rates in humans and other warm-blooded animals. Their importance is brought into sharp focus by cases of abnormal thyroid secretion.
The synthesis of thyroid hormones requires iodine, which is obtained from the diet. In the absence of iodine, blood levels of these hormones decrease. The anterior pituitary responds to this by secreting a thyroid stimulating hormone (TSH). Because the thyroid hormones cannot be synthesized, the feedback signal continues and so does TSH secretion. Excess TSH overstimulates the thyroid gland and causes it to enlarge. The enlargement is a form of goiter. Goiter caused by iodine deficiency is no longer common in countries where people use iodized salt.
Hypothyroidism results from insufficient concentrations of thyroid hormones. Hypothyroid adults often are overweight, sluggish, dry-skinned, intolerant of cold, and sometimes confused and depressed. Affected women often have menstrual disturbances. Hyperthyroidism results from excess concentrations of thyroid hormones. Hyperthyroid adults show an increased heart, rate, elevated blood pressure, weight loss despite normal caloric intake, intolerance of heat, and profuse sweating. Typically they are nervous, agitated, and have trouble sleeping.
Parathyroid functioning
Some endocrine glands do not respond directly to other hormones or nerves. They respond homeostatically to chemical change in the immediate surroundings. The parathyroid glands are like this. Four such glands are positioned next to the back of the human thyroid. They secrete parathyroid hormone (PTH) when the concentration of calcium ions in their surroundings decreases. Their action affects how much calcium is available for enzyme activation, muscle contraction, blood clotting, and many other tasks.
PTH stimulates bone cells to release calcium and phosphate ions, and stimulates the kidneys to conserve them. PTH also helps activate vitamin D. The activated form, a hormone, enhances calcium absorption from ingested food. In vitamin D deficiency, too little calcium and phosphorus is absorbed, so bones develop improperly. This ailment is called rickets.
Hormonal responses to the external environment influence growth, development, and reproduction in predictable ways. The pineal gland, a photosensitive organ in the brain, provides a good example of this. In the absence of light, the pineal gland secretes the hormone melatonin. Thus bood levels of melatonin vary from day to night, and they vary with the changing seasons. The variations influence the development of primary reproductive cycles and reproductive behavior in a variety of species.
Consider the hamster. In winter, when nights are longest, melatonin levels are high and sexual activity is suppressed. In summer, when days are longer, melatonin levels are low-and hamster sex peaks. Or consider a male white-throated sparrow. Melatonin suppresses growth of the bird’s gonads until spring, when days start to lengthen. Then, stepped-up gonadal activity leads to the production of hormones that influence singing behavior. With his distinctive song, the sparrow defines his territory and may hold the interest of a mate.
Does melatonin also influence human behavior? Perhaps, it seems possible that decreased melatonin secretion triggers puberty, the age at which human reproductive organs and structures start to mature. In cases where disease has destroyed the pineal gland, puberty begins prematurely, the focus essay mentions some other interesting possibilities.
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