Hyperthyroidism is the term for overactive tissue within the thyroid gland causing an overproduction of thyroid hormones (thyroxine or "T4" and/or triiodothyronine or "T3"). Hyperthyroidism is thus a cause of thyrotoxicosis, the clinical condition of increased thyroid hormones in the blood. It is important to note that hyperthyroidism and thyrotoxicosis are not synonymous. For instance, thyrotoxicosis could instead be caused by ingestion of exogenous thyroid hormone or inflammation of the thyroid gland, causing it to release its stores of thyroid hormones. Thyroid hormone is important at a cellular level, affecting nearly every type of tissue in the body.
Thyroid hormone functions as a controller of the pace of all of the processes in the body. This pace is called metabolism. If there is too much thyroid hormone, every function of the body tends to speed up. It is therefore not surprising that some of the symptoms of hyperthyroidism are nervousness, irritability, increased perspiration, heart racing, hand tremors, anxiety, difficulty sleeping, thinning of the skin, fine brittle hair, and muscular weakness—especially in the upper arms and thighs. More frequent bowel movements may occur, but diarrhea is uncommon. Weight loss, sometimes significant, despite a good appetite may occur, vomiting, and, for women, menstrual flow may lighten and menstrual periods may occur less often. Thyroid hormone is critical to normal function of cells. In excess, it both overstimulates metabolism and exacerbates the effect of the sympathetic nervous system, causing "speeding up" of various body systems and symptoms resembling an overdose of epinephrine (adrenaline). These include fast heart beat and symptoms of palpitations, nervous system tremor such as of the hands and anxiety symptoms, digestive system hypermotility (diarrhea), considerable weight loss and unusually low lipid panel (cholesterol) levels as indicated by a blood test.
Hyperthyroidism usually begins slowly. At first, the symptoms may be mistaken for simple nervousness due to stress. If one has been trying to lose weight by dieting, one may be pleased with weight loss success until the hyperthyroidism, which has quickened the weight loss, causes other problems.
In Graves disease, which is the most common form or cause of hyperthyroidism, the eyes may look enlarged because the eye muscles swell and push the eye forward. This can only be resolved surgically by orbital decompression. Sometimes, one or both eyes may bulge. Some patients have swelling of the front of the neck from an enlarged thyroid gland (a goiter). Because hyperthyroidism, especially Graves’ disease, may run in families, examinations of the members of a family may reveal other individuals with thyroid problems.
On the other hand, a lack of functioning thyroid tissue results in a symptomatic lack of thyroid hormone, termed hypothyroidism. Hyperthyroidism often eventually leads to hypothyroidism.
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