Thyroid Diseases
The thyroid is a small, butterfly-shaped gland located just below the Adam's apple. This gland plays a very important role in controlling your body's metabolism, that is, the rate at which your body uses energy. It does this by producing thyroid hormones (primarily thyroxine, or T4, and triiodothyronine, or T3), chemicals that travel through your blood to every part of your body. These thyroid hormones tell the cells in your body how fast to use energy and create proteins. The thyroid gland also makes calcitonin, a hormone that helps to regulate calcium levels in the blood by inhibiting the breakdown (reabsorption) of bone and increasing calcium excretion from the kidneys.
The body has an elaborate feedback system to control the amount of T4 and T3 in the blood. When blood levels decrease, the hypothalamus (an organ located in the brain) releases thyrotopin-releasing hormone, which in turn causes the pituitary gland (a tiny gland located below the hypothalamus and almost in the center of the head) to release thyroid-stimulating hormone (TSH). TSH stimulates the thyroid gland to produce and secrete thyroid hormones. When there is sufficient thyroid hormone in the blood, the amount of TSH decreases to maintain constant amounts of thyroid hormones, T4 and T3.
Inside the thyroid, most of the T4 is stored bound to a protein called thyroglobulin. When the need arises, the thyroid gland creates more T4 and/or releases some of what is stored. In the bloodstream, most T4 is bound to a protein called thyroxine-binding globulin (TBG) and is relatively inactive. T4 is converted to T3 by the liver and in many other tissues. T3 is primarily responsible for controlling the rate of body functions.
Thyroid diseases are primarily conditions that affect the amount of thyroid hormones being produced. Some create too few, leading to hypothyroidism and a slowing of body functions. This hypothyroidism causes symptoms such as weight gain, dry skin, constipation, cold intolerance, puffy skin, hair loss, fatigue, and menstrual irregularity in women. Severe untreated hypothyroidism, called myxedema, can lead to heart failure, seizures, and coma. In children, hypothyroidism can stunt growth and delay sexual development. In infants, it can cause mental retardation. For this reason, hypothyroidism testing is performed in the United States as part of newborn blood screening programs since early detection and treatment can minimize long-term damage.
If a thyroid disorder creates excessive amounts of thyroid hormone, the result is hyperthyroidism and the acceleration of body functions. This can lead to symptoms such as increased heart rate, anxiety, weight loss, difficulty sleeping, tremors in the hands, weakness, and sometimes diarrhea. There may be puffiness around the eyes, dryness, irritation, and, in some cases, bulging of the eyes. The affected person may experience light sensitivity and visual disturbances. Because the eyes may not move normally, the person may appear to be staring.
