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A number of conditions, including Graves' disease, toxic adenoma, Plummer's disease (toxic multinodular goiter) and thyroiditis, can cause hyperthyroidism.

Your thyroid is a butterfly-shaped gland situated at the base of your neck, just below your Adam's apple. Although it weighs less than an ounce, the thyroid gland has an enormous impact on your health. Every aspect of your metabolism is regulated by thyroid hormones.

Your thyroid gland produces two main hormones, thyroxine (T-4) and triiodothyronine (T-3), that influence every cell in your body. They maintain the rate at which your body uses fats and carbohydrates, help control your body temperature, influence your heart rate, and help regulate the production of protein. Your thyroid also produces calcitonin, a hormone that helps regulate the amount of calcium in your blood.

How it all works

The rate at which T-4 and T-3 are released is controlled by your pituitary gland and your hypothalamus — an area at the base of your brain that acts as a thermostat for your whole system. Here's how the process works:

The hypothalamus signals your pituitary gland to make a hormone called thyroid-stimulating hormone (TSH). Your pituitary gland then releases TSH — the amount depends on how much T-4 and T-3 are in your blood. If you don't have enough T-4 and T-3 in your blood, your TSH will rise; if you have too much, your TSH level will fall. Finally, your thyroid gland regulates its production of hormones based on the amount of TSH it receives. If the thyroid gland is diseased and is releasing too much thyroid hormone on its own, the TSH blood level will remain below normal; if the diseased thyroid gland cannot make enough thyroid hormone, the TSH blood level will remain high.

Reasons for too much thyroxine (T-4)

Normally, your thyroid releases the right amount of hormones, but sometimes it produces too much T-4. This may occur for a number of reasons, including:

  • Graves' disease. Graves' disease, an autoimmune disorder in which antibodies produced by your immune system stimulate your thyroid to produce too much T-4, is the most common cause of hyperthyroidism. Normally, your immune system uses antibodies to help protect against viruses, bacteria and other foreign substances that invade your body. In Graves' disease, antibodies mistakenly attack your thyroid and occasionally attack the tissue behind your eyes (Graves' ophthalmopathy) and the skin, often in your lower legs over the shins (Graves' dermopathy). Scientists aren't sure exactly what causes Graves' disease, although several factors — including a genetic predisposition — are likely involved.
  • Hyperfunctioning thyroid nodules (toxic adenoma, toxic multinodular goiter, Plummer's disease). This form of hyperthyroidism occurs when one or more adenomas of your thyroid produce too much T-4. An adenoma is a part of the gland that has walled itself off from the rest of the gland, forming noncancerous (benign) lumps that may cause an enlargement of the thyroid. Not all adenomas produce excess T-4, and doctors aren't sure what causes some to begin producing too much hormone.
  • Thyroiditis. Sometimes your thyroid gland can become inflamed for unknown reasons. The inflammation can cause excess thyroid hormone stored in the gland to leak into your bloodstream. One rare type of thyroiditis, known as subacute thyroiditis, causes pain in the thyroid gland. Other types are painless and may sometimes occur after pregnancy (postpartum thyroiditis).
Nov. 20, 2012