Thyroid and Parathyroid Glands The thyroid gland is located at the base of the neck and wraps around the upper part of the trachea. The thyroid gland has a major role in regulating the body's metabolism. Recall that metabolism is the sum of all the chemical reactions that occur in the body. The thyroid gland produces the hormone thyroxine, which increases the metabolic rate of cells throughout the body. Under the influence of thyroxine, cells become more active, use more energy, and produce more heat.

Iodine is needed to produce thyroxine. In parts of the world where diets lack iodine, severe health problems may result. Low levels of thyroxine in iodine-deficient infants produce a condition called cretinism (KREE tuh niz um), in which neither the skeletal system nor the nervous system develops properly. Iodine deficiency usually can be prevented by the addition of small amounts of iodine to table salt or other food items.

FIGURE 34–9 Thyroid Gland A goiter is an enlargement of the thyroid gland. A goiter may be the result of iodine deficiency. Without iodine, the thyroid cannot finish producing thyroxine, but its precursor continues to build up in the gland.

Thyroid problems are a fairly common disorder. If the thyroid produces too much thyroxine, a condition called hyperthyroidism occurs. Hyperthyroidism results in nervousness, elevated body temperature, increased blood pressure, and weight loss. Too little thyroxine causes a condition called hypothyroidism. Lower body temperature, lack of energy, and weight gain are signs of this condition. A goiter, as shown in Figure 34–9, can be a sign of hypothyroidism.

The thyroid also produces calcitonin, a hormone that reduces blood calcium levels. Calcitonin signals the kidneys to reabsorb less calcium from filtrate, inhibits calcium's absorption in the small intestine, and promotes calcium's absorption into bones. Its opposing hormone is parathyroid hormone, which is released by the four parathyroid glands located on the back surface of the thyroid. Parathyroid hormone (PTH) increases the calcium levels in the blood by promoting the release of calcium from bone, the reabsorption of calcium in the kidneys, and the uptake of calcium from the digestive system. The actions of PTH promote proper nerve and muscle function and proper bone structure.

In Your Notebook Summarize how blood-calcium levels are regulated.

Reproductive Glands The gonads—ovaries and testes—are the body's reproductive glands. The gonads serve two important functions: the production of gametes and the secretion of sex hormones. In females, ovaries produce eggs and secrete a group of hormones called estrogens. In males, the testes produce sperm and secrete the hormone testosterone. You'll learn more about the gonads and their hormones in the next lesson.

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