The major groups of hormones are amine hormones, peptide and protein hormones, and steroid hormones. Amine hormones are relatively small molecules that are structurally similar to amino acids. Epinephrine and norepinephrine, serotonin, dopamine, the thyroid hormones, and melatonin are examples of amine hormones. Peptide hormones and protein hormones are chains of amino acids. The peptide hormones have 3 to 49 amino acids, while the protein hormones are larger with chains of 50 to 200 or more amino acids. Examples of peptide hormones are antidiuretic hormone and oxytocin. The larger thyroid-stimulating hormone and follicle- stimulating hormone are examples of protein hormones. Steroid hormones are derived from cholesterol. Cortisol and the reproductive hormones (androgens in males and estrogens in females) are examples of steroid hormones. Most endocrine glands continue to function and secrete hormones throughout an individual’s lifetime. The most noticeable change in hormonal output is in the reproductive hormones. The ovaries decrease in size and no longer respond to FSH and LH, resulting in a decrease in the output of estrogens. Although the hormonal levels of other hormones may not change with aging and remain within normal limits, some endocrine tissues become less sensitive to stimulation. For example, elderly people may not produce as much insulin after a carbohydrate-rich meal is eaten. It has been suggested that the decrease in function of the immune system is a result of the reduced size of the thymus gland.
