Hyperthyroidism

  • Basics

    Hyperthyroidism is a condition in which the thyroid gland over-produces the hormones that regulate the body's metabolism. The excess thyroid hormones cause many symptoms. While hyperthyroidism responds well to treatment, there may be complications, and recurrence is common Figure 01. The thyroid is a hormone-secreting gland located in the neck that produces two important hormones: thyroxine and triiodothyronine. These hormones promote normal growth and development, and help to control the body's metabolism.

    Normally, the production of thyroxine and triiodothyronine is regulated by the pituitary gland. When blood levels of thyroid hormones are low, the pituitary increases production of thyrotropin, which is also known as thyroid-stimulating hormone (TSH). TSH in turn stimulates the thyroid to secrete thyroxine and triiodothyronine. Conversely, high blood levels of thyroid hormones signal the pituitary to turn off production of TSH, which in turn stops production of the thyroid hormones. This “feedback loop” controls the levels of thyroid hormones in the blood.

    Click to enlarge: Glands of the endocrine system

    Figure 01. Glands of the endocrine system

    Graves' disease, in which the whole gland becomes overactive, is the most common type of hyperthyroidism. Approximately 70% to 90% of people with hyperthyroidism have Graves' disease. In the disorder, the entire thyroid gland becomes enlarged and overactive. The enlarged gland appears as a bulge in the neck (goiter). The generalized overactivity of the thyroid in Graves' disease distinguishes it from another type of hyperthyroidism in which one or more lumps (nodules) in the thyroid become overactive, a condition called toxic nodule, or multinodular goiter. Graves' disease is approximately four times more common in women than men; the reason for this difference is not well understood.

    Hyperthyroidism occurs in about 0.5% of the population, and is more common in women than in men. The incidence of hypothyroidism increases with age. Only 5% of hyperthyroid patients are diagnosed before age 15, and the condition is most frequently identified in people between the ages of 30 and 40.

  • Causes

    Hyperthyroidism is a condition that occurs when the body produces too much thyroid hormone Table 01. Overstimulation of the thyroid may be caused by several different factors, acting at the thyroid itself (as is the case with Graves' disease and multinodular goiter), or less commonly, at the pituitary gland (as is the case in one type of pituitary tumor). The pituitary produces excess TSH, which in turn causes the thyroid to produce excess amounts of thyroid hormones.

    Table 1.  Causes of Hyperthyroidism

    Graves' disease (autoimmune stimulation of the thyroid gland)
    Hyperactivity of solitary or multiple thyroid nodules
    Inflammation of the thyroid gland (e.g., associated with infection or pregnancy)
    Intake of excessive amounts of thyroid hormones
    Excessive production of thyroid-stimulating hormone (TSH) by the pituitary gland

    Adapted from Hershman JM, Thyroid Dysfunction, Current Practice of Medicine 1999; 2(10).

    Graves' disease is a disorder in which antibodies stimulate the thyroid gland. A specific type of antibody is found in the blood of people with Graves' disease. This antibody stimulates the entire thyroid gland to grow and produce excess amounts of thyroid hormone. This type of hyperthyroidism tends to run in families, but little is known about the way it is inherited, or about other specific causes of the disease. Although the direct cause of Graves' disease is specific antibodies circulating in the blood, physicians have long thought that the condition may be triggered by severe emotional stress, such as the loss of a loved one. Studies have shown that stress can affect the ability of the immune system to function, so such a link is certainly possible. However, many patients develop Graves' disease without having identifiable stress in their lives.

    Hyperthyroidism may be caused by overactivity of solitary or multiple benign nodules within the thyroid. Individual or multiple nodules within the thyroid may enlarge and begin producing excess amounts of thyroid hormone. These nodules produce hormones, even in the absence of TSH, and as such are not affected by the normal mechanisms that control thyroid hormone levels.

    Some women experience hyperthyroidism after giving birth. For reasons not well understood, some women experience postpartum inflammation of the thyroid (thyroiditis). This inflammation appears to be caused by antibodies that attack the thyroid and cause a leakage of thyroid hormone into the blood. Although an estimated 5% to 7% of women will develop the condition after giving birth, it frequently goes undiagnosed, perhaps because the symptoms are attributed to the normal recovery process after childbirth.

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