Page 387



Page 387

Hormones that affect metabolism

1. Decreased secretion of growth hormone before puberty could cause dwarfism. In adults, it is believed to happen before aging because cell repair and protein production slows down.

2. If the production of growth hormone continues after the cartilaginous growth plates have been fused, other bones respond. Once the growth plates have fused, the long bones can no longer increase in length, but bones of the jaw, forehead, fingers, and toes increase in width. The disorder, referred to as acromegaly, causes a broadening of facial features.

3. Thyroxine increases metabolic rate. Individuals with high levels of thyroxine oxidize sugars and other nutrients at a faster rate. By increasing carbohydrate utilization, blood sugar levels can drop.

4. Hypothyroidism:

• lower metabolic rates; a tendency to gain weight

• lower levels of energy expenditure

• tend to get cold easily

• fatigue quickly, weakness

• depression

• dry, cold skin

Hyperthyroidism:

• increased metabolic rate, causing a tendency to reduced body fat

• higher levels of energy expenditure

• tend to remain warm even on cold days; warm, moist skin

• fatigue less easily

• fast heart rate

• nervousness, insomnia

• breathlessness

5. When the metabolic rate decreases, receptors in the hypothalamus are activated. Nerve cells release thyroid-releasing factor (TRF), which stimulates the pituitary to release thyroid-stimulating hormones (TSH). TSH is carried by way of the blood to the thyroid gland, which in turn releases thyroxine. The hormone increases metabolism by stimulating increased sugar utilization by the cells of the body. Higher levels of thyroxine cause the pathway to be “turned off.” Thyroxine inhibits the release of thyroid-releasing factor from the hypothalamus, thereby turning off the production of TSH from the pituitary.

6. A goiter is an enlargement of the thyroid gland due to a lack of iodine in a person’s diet. Goiters are located in the neck area and can interfere with the trachea and esophagus, creating difficulty in breathing, a cough, or a change in voice. Goiters can also compress the blood vessels of the neck.

7. Low calcium levels in the blood stimulate the release of parathyroid hormone (PTH). A rise in PTH levels causes the calcium levels in the blood to increase and phosphate levels to decrease. The hormone does this by acting on three different organs: the kidneys, the intestines, and the bones. PTH causes the kidneys and the intestines to retain calcium, and stimulates the release of calcium from the bones. Bone cells begin to break down bone and release calcium into the blood (the calcium is separated from phosphate ions). Calcium from the bones is reabsorbed into the blood, while the phosphate is excreted. PTH also increases the absorption of calcium from undigested food. Once calcium levels have increased, the level of PTH is inhibited.

8. Rapid, uncontrolled muscle twitching referred to as tetany, indicates low calcium levels. If the parathyroid glands are removed, calcium levels fall and phosphate levels rise. Without the parathyroid glands, PTH is not produced, so calcium is not reabsorbed in the gut or released from bones.

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