The major endocrine glands of mammals include the hypothalamus, pituitary gland, thyroid gland, parathyroid glands, pancreas, adrenal glands, gonads, and pineal gland. Each of these glands secretes and responds to hormones that play crucial roles in controlling physiology and development. Thyroxine is a lipid-soluble hormone that influences the expression of genes involved in regulating cell metabolism. Calcitonin and parathyroid hormone (PTH) regulate blood calcium levels, mainly by regulating the remodeling of bone. Blood glucose levels are controlled by insulin when blood glucose levels rise and largely by the lack of insulin when blood glucose levels fall. Glucagon is released in the event of serious hypoglycemia, and it stimulates breakdown of glycogen to raise blood glucose levels. The adrenal cortex produces steroid hormones, the most important of which is cortisol, which is involved in stress responses. The adrenal medulla produces epinephrine and norepinephrine.

Question 1

A major cause of hypothyroidism is lack of iodine. Without adequate iodine, the thyroid cannot secrete active T₃ and T₄. As a result there is no negative feedback on the production of TRH and TSH. The elevated TSH causes increased production of thyroglobulin and growth of the thyroid gland. A major cause of hyperthyroidism is production of an antibody to the TSH receptor that binds to the receptor and keeps it activated. The result is continuous stimulation of thyroglobulin production. Even though the individual produces active T₃ and T₄, they cannot exert negative feedback on the hypothalamic–pituitary–thyroid system.

Question 2

Vitamins are required in the diet because they are not produced in the body. Since vitamin D is produced in the body by the action of UV light on a precursor in the skin, converting it to calciferol, it is not a vitamin. Calciferol is converted to the active form calcitriol by enzymes in the liver and kidneys. Calcitriol (vitamin D) promotes the uptake of calcium from the gut and the kidneys, thereby raising blood calcium levels.

Question 3

Insulin controls the rate of glucose uptake by cells by stimulating the cells to insert glucose transporters (passive transporters) into the cell membrane from a pool of those receptors in the cytoplasm. The glucose transporters in the cell membrane increase the permeability of the cell to glucose, which can then diffuse into the cell as long as its concentration in the extracellular fluid is high enough.

Question 4

Stress stimulates the release of cortisol from the adrenal cortex. Cortisol helps the body deal with short-term stress by reducing cellular uptake of glucose, elevating blood pressure, inhibiting activity of the gut and the reproductive system, and even inhibiting the activity of the immune system. In the short run, these responses make more energy available to deal with the immediate stressor. Negative feedback from cortisol, acting through the hypothalamus and the pituitary gland normally shuts off the cortisol response to short-term stress. If, however, stress becomes chronic, as in a difficult job situation, cortisol levels remain high and the resulting higher blood pressure can lead to heart disease, the impaired immune system can lead to infections and diseases, the impaired digestion can lead to ulcers, and the impaired glucose metabolism can lead to diabetes.