Pinealocytes, the parenchymal cells of the pineal gland, produce melatonin at night and serotonin during the day. The pineal gland also contains neuroglial cells that resemble astrocytes, and its interstitium has calcified concretions called brain sand.

Protein hormones initiate their action by binding externally to transmembrane receptor proteins in the target-cell plasma membrane. Receptors for some hormones (e.g., TSH, serotonin, epinephrine) are linked to G proteins; other receptors, including those for insulin and growth hormone, have protein kinase activity.

A. Calcitonin lowers blood calcium levels and thus has an effect antagonistic to that of parathyroid hormone. It is produced by parafollicular cells of the thyroid gland.

Graves disease (exophthalmic goiter) results in an enlarged thyroid gland due to stimulation of the follicular cells to produce an excessive amount of thyroid hormones by binding of auto- immune antibodies to TSH receptors. Follicular cells actively remove colloid from the lumen of the follicles. Heat intolerance and weight loss are common, but the disease is not caused by iodine deficiency.

E. Triiodothyronine (T3) and thyroxine (T₁) both increase the basal metabolic rate, which affects heat production and body temperature. These thyroid hormones also have many other effects.

C. Upon removal of the carcinoma from his neck, the parathyroid glands were also removed or damaged, causing hypoparathyroidism (a lack of parathyroid hormone that increases blood calcium). Treatment with calcium (and vitamin D, which aids in its absorption) corrected these symptoms. The marked neuromuscular irritability in the absence of calcium reveals its importance in regulating skeletal muscle contraction.

Stimulation of the thyroid gland by TSH causes the follicular cells to become more active and column shaped. They form apical pseudopods and engulf colloid, which is removed from the lumen of the follicle by endocytosis and broken down by controlled lysosomal hydrolysis to yield the thyroid hormones T3 and T₁. Parafollicular cells and capillaries do not contain receptors for TSH.

ACTH is produced by corticotrophs, a type of basophil, present in the pars distalis of the pituitary gland.

Chromaffin cells in the adrenal medulla synthesize and store epinephrine and norepinephrine in secretory granules, which also contain ATP, chromogranins, and enkephalins. The cortical parenchymal cells of the zona fasciculata produce glucocorticoids, and those of the zona glomerulosa produce mineralocorticoids. The cortical parenchymal cells do not store their secretory products and thus do not contain secretory granules.

Prolactin is produced by mammotrophs, one of the two types of acidophils located in the pars distalis of the pituitary gland. As their name implies, these cells produce a hormone that regulates the development of the mammary gland during pregnancy and lactation.