a. Increase or decrease the activities of other glands.
b. Stimulate or retard the metabolism in general.
c. Influence the growth or development of the whole body or a particular part of
it. Hormones have a profound effect on growth, development, physical and mental
wellbeing, and personality. When hormones are absent from the body, due to
malfunction of a gland or any other reason, or when they are manufactured and
secreted in excess due to hypertrophy of an organ, serious disturbances and diseases
The thyroid is bilobate and lies in front of and on either side of the trachea. It
secretes the hormone thyroxine, which has several functions, including: (1) speeding
up cell metabolism, (2) speeding up heart rate, (3) depleting fat stores, and (4)
increasing motility of the gastrointestinal tract. Thyroxine is formed from iodine and
tyrosine (an amino acid). When iodine is lacking in the diet, the cells of the thyroid
begin to hypertrophy to compensate for the decreased production of thyroxine. The end
result is a goiter. A goiter can also result from an overactive thyroid. In this instance,
an overproduction of thyroxine (hyperthyroidism) occurs, the person is very nervous,
has a high metabolic rate, and may even exhibit exophthalmos (protruding eyeballs). In
addition, the thyroid gland secretes the hormone calcitonin, which acts as a shortterm
regulator of the blood level of calcium ion. The presence of calcitonin reduces the
Attached to the thyroid gland, but quite small in size, are the parathyroids. The
parathyroids secrete parathormone, which is primarily responsible for regulating calcium
levels in the extracellular fluids (interstitial and vascular fluids). The chief cells within
the parathyroid gland are thought to be the cells that elaborate parathormone. A proper
level of calcium ions is needed in the circulatory system for the blood clotting
mechanism to function normally. In addition, a specific level of calcium ions is required
in the extracellular fluids to prevent tetany or the continuous spasmotic contraction of
skeletal muscle groups. It is also thought that parathormone is responsible for
promoting kidney excretion of phosphate. If the serum concentration of calcium falls
below a critical point, parathormone will be secreted in large amounts and will dissolve
bone to raise the calcium level in blood.
The thymus is located anterior to the aorta and posterior to the sternum. The
thymus is probably the primary source of lymphocytes at birth, but later the lymphatic
system is the chief source of lymphocytes. In the adult, cells of the thymus are thought
to be involved in antibody production.