What do you know about Thyroid and Parathyroid Glands

Thyroid and Parathyroid Glands 
The thyroid gland is located in the neck and is attached to the trachea just inferior to the larynx. The parathyroid glands are embedded in the posterior surface of the thyroid gland.
Thyroid Gland 
The thyroid gland is composed of a large number of folli­cles, each a smalL spherical structure made of thyroid cells and filled with stored thyroxine. Thyroxine, one hormone produced by the thyroid gland, occurs in two forms. Thy­roxine is usually secreted as T4 (tetraiodothyronine), which contains four iodine atoms, but eventually this form is converted to T3 (triiodothyronine), the active form of the hormone. Iodine, which is required for thyroxine production, is actively transported into the thyroid gland, where its concentration may be as much as 25 times that found in the blood. If iodine is lacking in the diet, the thy­roid gland enlarges, producing a simple goiter. The use of iodized salt helps prevent such a condition.
When the level of thyroxine in the blood is low (called hypothyroidism), the anterior pituitary pro­duces TSH, which stimulates the thyroid. If iodine is not present in the thyroid, the level of thyroxine will remain the same, and TSH will continue to stimulate the thyroid. The result is hypertrophy of the gland, called a simple goiter.
Thyroxine increases the metabolic rate. It does not have one target organ; instead, it stimulates most of the cells of the body to metabolize at a faster rate. For example, it causes more glucose to be broken down.
Failure of the thyroid to develop properly results in a condition called cretinism (kre'tI-nizm). Cretins are short and stocky, and have had extreme hypothyroidism since childhood or infancy. Thyroxine therapy can initiate growth, but unless treatment is begun within the first two months of birth, mental retardation results. Hypothy­roidism in adults produces the condition known as myxedema (mik"se-de'mah), which is characterized by lethargy, weight gain, loss of hair, slower pulse rate, low­ered body temperature, and thickness and puffiness of the skin. The administration of adequate doses of thyroxin re­stores normal function and appearance.
In the case of hyperthyroidism, or Graves' (gravz) disease, the thyroid gland is enlarged and overactive, causing a goiter to form and the eyes to protrude because of edema in eye socket tissues and swelling of extrinsic eye muscles. This type of goiter is called exophthalmic (ek"sof-thal'mik) goiter. The patient usually becomes hyperactive, nervous, and irritable, and suffers. from insomnia. Hyperthyroidism can also be caused by a thyroid tumor, which is usually detected as a lump during physical examination. The treatment for hyperthyroidism is surgery in combination with administration of radioac­tive iodine. The prognosis for most patients is excellent.
Calcitonin 
In addition to thyroxine, the thyroid gland also produces the hormone calcitonin (kal"sI-to'nin), which helps reg­ulate the calcium level in the blood and opposes the ac­tion of parathyroid hormone. (The interaction of these two hormones is discussed in the next section.) Calci­tonin lowers blood calcium by increasing the buildup of bone.
The anterior pituitary produces TSH, a hormone that promotes the production of thyroxine by the thyroid. Thyroxine, which increases metabolism, can affect the 4imtire body, as exemplified by cretinism and myxedema. The thyroid also produces calcitonin, which lowers the blood calcium level.
Parathyroid Glands 
Many years ago, the four, small parathyroid glands were sometimes mistakenly removed during thyroid surgery. Parathyroid hormone (PTH), the hormone produced by the parathyroid glands, causes the calcium (Ca2+) level in the blood to increase and the phosphate (HPO4 -2) level to decrease. PTH promotes bone breakdown and calcium re­tention by the kidneys, and activates vitamin D, which, turn, stimulates the absorption of calcium from the intes­tine. It also promotes the kidneys' excretion of phosphate in unne.
Parathyroid hormone inhibits the activity of os­teoblasts and promotes the activity of osteoclasts in bone, thereby raising the blood calcium level. Calcitonin has the opposite effect, and therefore, the homeostatic balance of calcium in the blood is achieved through the action of both hormones.
If insufficient parathyroid hormone is produced, the blood calcium level drops, resulting in tetany (tet'ah-ne). In tetany, the body shakes from continuous muscle con­traction. The effect is actually brought about by increased excitability of the nerves, which fire spontaneously and without rest. Calcium plays an important role in both ner­vous conduction and muscle contraction. It is also neces­sary to blood clotting.
PTH maintains a high blood calcium level by promoting calcium absorption in the intestine, calcium retention by the kidneys, and bone breakdown. These actions are opposed by calcitonin produced by the thyroid gland.
Adrenal Glands 
The adrenal glands, as their name implies (ad means near; renal means kidneys), lie atop the kidneys. Each consists of an outer portion, called the cortex, and an inner portion, called the medulla. These portions, like the anterior and posterior pituitaries, have no connection with one another.
The hypothalamus exerts control over the activity of both portions of the adrenal glands. It can initiate nerve impulses that travel by way of the brain stem, spinal cord, and sympathetic nerve fibers to the adrenal medulla, which then secretes its hormones. The hypothalamus, by means of corticotrophin-releasing hormone, controls the anterior pituitary's secretion of adrenocorticotropic hormone (ACTH), which, in turn, stimulates the adrenal cortex. Stress of all types, including both emotional and physical trauma, prompts the hypothalamus to stimulate the adre­nal glands to release hormones.
Adrenal Medulla 
The adrenal medulla, which is under the control of the sympathetic division of the autonomic nervous system, produces epinephrine (ep"i-nefrin), also called adrena­line, and norepinephrine (nor"ep-i-nefrin), also called noradrenaline. These hormones are responsible for the "fight-or-flight" reaction that occurs in times of emer­gency. Epinephrine and norepinephrine bring about these reactions:

• Blood glucose level rises, and the metabolic rate in­creases. 
• Bronchioles dilate, and breathing rate increases. 
• Blood vessels to the digestive tract constrict; those to the skeletal muscles dilate. 
• Cardiac muscle contraction is more forcefuL and heart rate increases. 

The adrenal medulla releases epinephrine and norepinephrine into the bloodstream. helping the body cope vvith situations that seem to threaten survival.

Posted in Human Anatomy and Physiology