Hypothyroidism is a condition in which the level of the thyroid hormones in the body is abnormally low. The thyroid gland is the butterfly-shaped organ located at the base of the throat. It secretes two important hormones—T3 and T4—that the body needs to function well. These hormones regulate how quickly the body converts the nutrients from oxygen and food into energy. Heart rate, weight, body temperature, alertness, and mood are among the many things affected by thyroid hormones. When then thyroid gland becomes underactive, thyroid hormone levels dip.
Two other glands—the hypothalamus and the pituitary gland—work together with the thyroid to regulate thyroid hormone levels. There are many things that can interfere with the functioning of all these organs, and, consequently, thyroid hormone levels. For example, the pituitary gland, located in the brain, is one of the “master glands,” and controls other glands in the body. It makes a hormone called thyroid-stimulating hormone (TSH), which is involved in hypothyroidism. TSH instructs the thyroid gland to produce T3 and T4 when levels of those hormones are low. Likewise, if levels of T3 and T4 in the blood are too high, the pituitary gland “turns off” the thyroid gland.
The hypothalamus gland, another “master gland” of the brain, is similarly involved in thyroid hormone levels, providing a feedback loop to keep levels in check. If either of these glands is not functioning properly, then hypothyroidism results.
Hashimoto’s thyroiditis is the most common cause of hypothyroidism. Hashimoto’s thyroiditis occurs when a person’s own immune system attacks the thyroid gland, causing the gland to become inflamed initially, and then become underactive. Immune diseases can run in families, and are more common in women. Hashimoto’s thyroiditis is no exception, as it is particularly common in women—especially as they age. The disease is also linked to other autoimmune diseases, such as diabetes and rheumatoid arthritis.
Hypothyroidism can develop when treatments to correct an overactive thyroid cause the gland to shut down.
- Undergoing surgery or receiving radioactive iodine treatment, which might be done to keep the thyroid from churning out too much thyroid hormone, can disable the gland.
- Radiation therapy for certain types of cancer also commonly disables the thyroid gland, causing hypothyroidism.
- Medications such as lithium (Eskalith, Lithane, Lithobid, Cibalith-S) and amiodarone (Cordarone), as well as surgery for throat cancer or Hodgkin’s disease, can produce hypothyroidism.
Rarely, hypothyroidism stems from a problem with the pituitary gland. Pituitary tumors and other pituitary disorders can affect the hormonal chain of command that governs thyroid function, and result in underproduction of thyroid hormone.
Not getting enough dietary iodine is another cause of hypothyroidism, but is not commonly seen in the U.S. Iodine is a trace element that is needed to make thyroid hormone. In this country, iodine-deficiency hypothyroidism is rare, thanks to the introduction of iodized salt in the 1920s.
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