Overview of Hypothyroidism
Causes, Evaluation, and Treatment of a Hypoactive Thyroid Gland
Aug 30, 2008
The thyroid gland is the endocrine organ in the neck that regulates overall body metabolism. The thyroid hormone it produces stimulates cells to perform multiple necessary functions. However, if this gland is hypoactive (hypothyroidism) or hyperactive (hyperthyroidism), a variety of abnormalities may occur.
The following is an overview of the causes, clinical manifestations, diagnosis, and treatment of hypothyroidism.
Causes
The thyroid gland secretes thyroid hormone in response to thyroid-stimulating hormone (TSH) from the pituitary gland, which in turn is secreted in response to thyrotropin-releasing hormone (TRH) from the hypothalamus in the brain. Hypothyroidism can result from problems in the thyroid gland (primary), pituitary gland (secondary), or hypothalamus (tertiary).
Causes of primary hypothyroidism include autoimmune thyroiditis (Hashimoto's thyroiditis), postpartum thyroiditis, certain medications causing hypothyroidism as a side effect, overtreatment of hyperthyroidism, radiation to the neck, and iodine deficiency or excess. In contrast, causes of secondary and tertiary hypothyroidism, such as tumors in the pituitary gland or hypothalamus, are more rare.
Clinical Manifestations
Hypothyroidism is associated with effects across multiple body systems. Principally, hypothyroidism causes weakness, fatigue, decreased appetite, weight gain, and intolerance to cold. It also results in coarse hair, dry skin, and musculoskeletal pain. Mentally, hypothyroidism can impair the ability to concentrate and depress the mood.
For some patients, the condition is extreme and becomes life-threatening. Such hypothyroidism is called myxedema. This is characterized by coma, respiratory depression requiring mechanical ventilation, and significant hypothermia. A variety of factors can trigger myxedema, including but not limited to cold environment, infection, drugs affecting the nervous system, and trauma.
Diagnosis
The tests that are necessary to diagnose hypothyroidism are levels of TSH and total levels of both T4 and T3 thyroid hormone. Any cause of hypothyroidism results in low T4 and T3. With primary hypothyroidism, TSH levels are elevated because low T4 and T3 levels are unable to provide enough negative feedback to stop the production of TSH by the pituitary gland. In secondary and tertiary hypothyroidism, TSH is not likely to be increased.
If a physician suspects autoimmune thyroiditis, he or she may also check for autoimmune thyroid antibodies.
Treatment
The primary treatment of hypothyroidism is thyroid hormone replacement. Various doses of levothyroxine are available. In general, a starting dose is administered, and a TSH level is rechecked periodically to determine if dose changes are necessary. For patients with myxedema, thyroid hormone replacement is given intravenously.
There are also patients who have subclinical hypothyroidism, characterized by elevations in TSH levels but otherwise normal levels of T4 thyroid hormone and minimal to no signs of hypothyroidism. Such patients can either be treated or just be monitored; the decision is made on a case-by-case basis.
