Wednesday, September 2, 2009

Diseases of the Thyroid


Most effects of hyperthyroidism are obvious from the preceding discussion of the various physiologic effects of thyroid hormone. However, some specific effects should be mentioned in connection especially with the development, diagnosis, and treatment of hyperthyroidism.Causes of Hyperthyroidism (Toxic Goiter, Thyrotoxicosis, Graves’Disease).

In most patients with hyperthyroidism, the thyroid gland is increased to two to three times normal size, with tremendous hyperplasia and infolding of the follicular cell lining into the follicles, so that the number of cells is increased greatly. Also, each cell increases its rate of secretion severalfold; radioactive iodine uptake studies indicate that some of these hyperplastic glands secrete thyroid hormone at rates 5 to 15 times normal.The changes in the thyroid gland in most instances are similar to those caused by excessive TSH. However,plasma TSH concentrations are less than normal rather than enhanced in almost all patients and often are essentially zero. However, other substances that have actions similar to those of TSH are found inthe blood of almost all these patients. These substancesare immunoglobulin antibodies that bind with the same membrane receptors that bind TSH. They induce continual activation of the cAMP system of the cells,with resultant development of hyperthyroidism.
These antibodies are called thyroid-stimulating immunoglobulin and are designated TSI.They have a prolonged stimulating effect on the thyroid gland, lasting for as long as 12 hours, in contrast to a little over 1 hour for TSH.
The high level of thyroid hormone secretion caused by TSI in turn suppresses anterior pituitary formation of TSH.
The antibodies that cause hyperthyroidism almost certainly occur as the result of autoimmunity that has developed against thyroid tissue. Presumably, at some time in the history of the person, an excess of thyroid cell antigens was released from the thyroid cells, and this has resulted in the formation of antibodies against the thyroid gland itself.
Thyroid Adenoma. Hyperthyroidism occasionally results from a localized adenoma (a tumor) that develops in the thyroid tissue and secretes large quantities of thyroid hormone. This is different from the more usual type of hyperthyroidism, in that it usually is not associated with evidence of any autoimmune disease. An interesting effect of the adenoma is that as long as it continues to secrete large quantities of thyroid hormone, secretory function in the remainder of the thyroid gland is almost totally inhibited because the thyroid hormone from the adenoma depresses the production of TSH by the pituitary gland.

Symptoms of Hyperthyroidism

The symptoms of hyperthyroidism are obvious from thepreceding discussion of the physiology of the thyroidhormones:
(1)a high state of excitability,
(2)intolerance to heat,
(3)increased sweating,
(4)mild to extreme weight loss (sometimes as much as 100 pounds),
(5)varying degrees of diarrhea,
(6)muscle weakness,
(7)nervousness or other psychic disorders,
(8)extreme fatigue but inability to sleep, and
(9)tremor of the hands.


Most people with hyperthyroidism develop some degree of protrusion of the eyeballs. This condition is called exophthalmos.A major degree of exophthalmos occurs in about one third of hyperthyroid patients, and the condition sometimes becomes so severe that the eyeball protrusion stretches the optic nerve enough to damage vision. Much more often, the eyes are damaged because the eyelids do not close completely when the person blinks or is asleep. As a result, the epithelial surfaces of the eyes become dry and irritated and often infected, resulting in ulceration of the cornea. The cause of the protruding eyes is edematous swelling of the retro-orbital tissues and degenerative changes in the extraocular muscles.

In most patients,immunoglobulins can be found in the blood that react with the eye muscles. Furthermore, the concentration of these immunoglobulins is usually highest in patients who have high concentrations of TSIs.

Therefore, there is much reason to believe that exophthalmos, like hyperthyroidism itself, is an autoimmune process.The exophthalmos usually is greatly ameliorated with treatment of the hyperthyroidism.

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