CHAPTER 8 Endocrine System^333
regulated by a process called direct feedback. Direct feedback is necessary to main-
tain homeostasis. The body receives feedback about changes in hormone levels and
impacts organs or body systems to adjust the hormone production to tell the body to
return to homeostasis. When the concentration of the substance reaches a threshhold,
the gland and its production are turned off. The glands, the hormones they produce,
and the effects are stated below.
The thyroid gland is located in the anterior neck, overlying the trachea. It makes
three hormones: thyroxine (T4), triiodothyronine (T3), and calcitonin, which
affects the blood calcium and phosphate release from the bones. Thyroid hormones
affect metabolism, muscles, the heart, and many other body organs and systems.
They help regulate carbohydrate metabolism, lipids, proteins, and growth and
development. Anterior pituitary glands are controlled by hormones from the
hypothyroid and by direct feedback. The adrenal glands are bilateral glands that cap
each kidney. They are located in the retro peritoneum. The glands are comprised of
two parts: the cortex and the aderenal medulla. The cortex secretes (1) aldosterone,
which is responsible for renal reabsorption of sodium and excretion of potassium;
(2) cortisol, which maintains glucose control, increases hepatic gluconeogenesis (the
making of glucose), and manages the body’s stress response; and (3) androgens,
which are sex hormones. The adrenal medulla produces, stores, and secretes epi-
nephrine and norepinephrine, which are called catecholamines. When they are
released, heart and respiratory rate increase, blood pressure rises, airways dilate, and
an increase in the metabolic rate is seen.
The parathyroid glands are composed of usually four, sometimes six or more,
small glands which are found on the posterior side of the thyroid gland. Their func-
tion is to produce parathyroid hormone (PTH). PTH, also called parathormone,
maintains the calcium level in the blood. It also regulates the phosphorus level
in the body. If the serum calcium level falls, PTH is released, which causes bones
to break down, releasing calcium into the blood. It also causes the kidneys to
decrease the calcium released in the urine, and increases phosphate excretion.
Just the Facts
Hypothyroidism (Myxedema)
WHAT WENT WRONG?
Hypothyroidism is a lack of, or too little, thyroid hormone commonly caused
by Hashimoto’s thyroiditis. Hashimoto’s thyroiditis is a chronic disorder caused by
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