Thyroid disorders are very common in the general U.S. population, affecting up to 27 million Americans, although half that number remains undiagnosed. It is second only to diabetes as the most common condition to affect the endocrine system — a group of glands that secrete hormones that help regulate growth, reproduction, and nutrient use by cells. As a result, it is common for an individual to be affected by both thyroid disease and diabetes.
Since the thyroid gland plays a central role in the regulation of metabolism, abnormal thyroid function can have a major impact on the control of diabetes. In addition, untreated thyroid disorder can increase the risk of certain diabetic complications and can aggravate many diabetes symptoms. Luckily, abnormal thyroid function can easily be diagnosed by simple blood tests, and effective treatment is available. For all of these reasons, periodic screening for thyroid disorder should be considered in all people with diabetes.
What is the thyroid?
The thyroid is a butterfly-shaped gland located in the neck, just below the Adam’s apple and above the collarbone. It produces two hormones, thyroxine (T4) and triiodothyronine (T3), which enter the bloodstream and affect the metabolism of the heart, liver, muscles, and other organs. The thyroid gland operates as part of a feedback mechanism involving the hypothalamus, an area of the brain, and the pituitary gland, which is located within the brain.
First, the hypothalamus sends a signal to the pituitary through a hormone called TRH (thyrotropin-releasing hormone). When the pituitary gland receives this signal, it releases TSH (thyroid-stimulating hormone) to the thyroid gland. Upon receiving TSH, the thyroid responds by producing and releasing the two thyroid hormones (T3 and T4). The pituitary gland “monitors” the level of thyroid hormone in the blood and increases or decreases the amount of TSH released, which in turns regulates the amount of thyroid hormone produced.
Function of the thyroid
Thyroid hormone regulates the way the body uses energy. It works by attaching to specific proteins called receptors that are present in cells throughout the human body. Therefore, thyroid hormone exerts wide-ranging effects in regulating the function of virtually every organ. Consequently, any changes in the blood level of thyroid hormone can affect many body systems and cause a wide range of symptoms.
The extent to which each organ is affected varies widely between individuals, which is why thyroid dysfunction causes very different symptoms in different people. In general, the severity of symptoms of abnormal thyroid function depends on the severity of the actual condition, the length of time it has been present, and the person’s age. As a result, it is difficult to correctly diagnose thyroid disorder based only on symptoms. Fortunately, precise measurement of thyroid function is now possible with the TSH blood test, a test that directly measures the amount of TSH produced by the pituitary gland.
Common disorders of the thyroid
There are two basic disorders of the thyroid: hypothyroidism, or an underactive thyroid gland, and hyperthyroidism, or an overactive thyroid gland.
Hypothyroidism. The most common disorder of the thyroid is an underactive thyroid gland, or hypothyroidism. Some studies have shown that up to 10% of women and 3% of men in the United States have hypothyroidism and receive thyroxine replacement therapy. Although hypothyroidism can occur at any age and in either sex, it is most common in older women. It is estimated that up to 1 in 5 women over the age of 65 may have hypothyroidism. At the other end of the age range, 1 out of every 4,000 babies in the United States is born without a properly functioning thyroid (congenital hypothyroidism). (For more information about populations in which thyroid disorders are more likely, see “Who’s At Risk?”)