Hyperthyroidism is defined as the excess production of thyroid hormone. This condition leads to several physiologic effects that may alter even the physical appearance of a patient with hyperthyroidism. Thyroid hormone controls most of the body's metabolism and this ability is made manifest in force in cases of hyperthyroidism.
What Causes Hyperthyroidism?
Patients with hyperthyroidism typically have thyroid glands that are two to three times larger than normal thyroid gland. These hyperthyroid glands are characterized by increased cell proliferation and infolding of the follicular cell lining into the follicles, increasing cell population even more. These hyperthyroid cells also secret thyroid hormone at a rate faster than normal thyroid cells.
These changes may also be found in instances where there is a large amount of circulating thyroid-stimulating hormone (TSH) in the body. However, in hyperthyroidism, TSH levels are decreased due to the inhibitory of their secretion by the already increased amount of circulating thyroid hormone in the body. In normal conditions, an increase of thyroid hormone signals the pituitary gland to stop secreting TSH and consequentially, the thyroid gland stops secreting thyroid hormone due to lack of stimulus by the decrease in TSH. This is not so in hyperthyroidism.
In hyperthyroidism, a similar substance to TSH may be found in circulation – thyroid-stimulating immunoglobulin or TSI. They have a prolonged stimulatory effect on the thyroid gland and causes thyroid cells to continue secret since the reduced levels of TSH.
Another cause for hyperthyroidism is the presence of a thyroid adenoma or a tumor in the thyroid tissue that uncontrollably secretes increased amounts of thyroid hormone. Normal thyroid cells around the tumor stop secreting hormone due to reduced TSH levels but the tumor keeps on secreting thyroid hormone by itself. This cause does not have any association with autoimmunity.
Symptoms of Hyperthyroidism
Hyperthyroidism is characterized by a high state of excitability, intolerance to heat, increased sweating, mild to extreme weight loss, diabetes, muscle weakness, nervousness, extreme fatigue but inability to sleep, and tremor of the hands.
Another symptom of hyperthyroidism is exophthalmos , in which there is protrusion of the eyeballs. In several cases, the degree of protrusion is so great that it stretches the optic nerve enough to damage it. The eyesballs also do not close completely when sleep or when the patient blinks, causing further damage to the eyes.
Diagnostic Tests for Hyperthyroidism
Hyperthyroidism is usually diagnosed based on the levels of free thyroxine circulating in the plasma using radioimmunoassay procedures. In some instances, the measure of triiodothyronine is also included in the workup.
Other tests or signs that indicate hyperthyroidism include:
- Increase in the metabolic rate of the patient by + 30 to +60 in cases of severe hyperthyroidism.
- Decrease in the concentration of TSH in the plasma. This is because the body tries to control the excessive secretion of thyroid hormone by suppressing its usual stimulant. In the usual type of thyrotoxicosis, there is very little plasma TSH left.
- Measurement of TSI levels to differentiate between thyrotoxicosis. TSI levels are typically increased in cases of thyrotoxicosis but low in cases of thyroid adenoma.
Treatment for Hyperthyroidism
Surgical removal of most of the thyroid gland is the most direct treatment for hyperthyroidism. Prior to the operation, the patient is first administrated with doses of propylthiouracil, which decrees hormone function, until the patient's metabolic rate returns to normal. Then, large doses of iodides are administered for one to two weeks immediately before the operation, causing the gland to recede in size and its blood supply to diminish. These procedures have reduced the operative mortality to 1 in 1000 operations from 1 in 25 prior to development of modern procedures.
In other cases of hyperthyroidism, a hyperplastic thyroid gland may also be treated with radioactive iodine. Eighty to ninety percent of the iodine injected is absorbed by the hyperplastic gland. Because the iodine is radioactive, it destroys almost all the secretory cells of the thyroid gland. Typically, the patient is administrated with 5 millicuries of radioactive iodine and assessed several weeks later. If the patient remains hyperthyroid, additional doses may be given until normal thyroid function is reinstated.