The thyroid is a small gland, measuring about 2 inches (5 centimeters) across, that lies just under the skin below the Adam’s apple in the neck. The two halves (lobes) of the bland are connected in the middle (called the isthmus), giving the thyroid gland the shape of a bow tie.
Normally, the thyroid gland cannot be seen and can barely be felt. If it becomes enlarged (goiter), doctors can feel it easily, and a prominent bulge may appear below or to the sides of the Adam’s apple
The thyroid bland secretes thyroid hormones, which control the speed at which the body’s chemical functions proceed (metabolic rate). Thyroid hormones influence the metabolic rate in two ways by stimulating almost every tissue in the body to produce proteins and by increasing the amount of oxygen that cells use. Thyroid hormones affect many vital body functions: the heart rate, the respiratory rate, the rate at which calories are burned, skin maintenance, growth, heat production, fertility, and digestion.
The two thyroid hormones are T4 (thyroxine) and T3 (triiodothyronine). T4, the major hormone produced by the thyroid gland, has only a slight, if any, effect on speeding up the body’s metabolic rate. Instead, T4 is converted into T3, the more active hormone. The conversion of T4 to T3 occurs in the liver and the tissues. Many factors control the conversion of T4 to T3, including the body’s needs from moment to moment and the presence or absence of illnesses. Most of the T4 and T3 in the bloodstream are carried bound to a protein called thyroxine-binding globulin. Only a little of the T4 and T3 are circulating free in the blood. However, it is this free hormone that is active. When the free hormone is used by the body, some of the bound hormone is released from the binding protein.
To produce thyroid hormones, the thyroid gland needs iodine, and element traps iodine and processes it into thyroid hormones. Thyroid hormones are used, some of the iodine contained in the hormones is released, returns to the thyroid gland, and is recycled to produce more thyroid hormones. Oddly, the thyroid gland releases slightly less of the thyroid hormones if it is exposed to high levels of iodine transported to it in the blood.
The body has a complex mechanism for adjusting the level of thyroid hormones. First, the hypothalamus, located just above the pituitary gland in the brain, secrets thyrotropin-releasing hormone, which causes the pituitary gland to produce thyroid-stimulating hormone (TSH). Just as the name suggests, TSH stimulates the thyroid gland to produce thyroid hormones. The pituitary gland slows or speeds the release of TSH, depending on whether the levels of thyroid hormones circulating in the blood are getting too high or too low.
The thyroid bland also produces the hormone calcitonin, which may contribute to bone strength by helping calcium to be incorporated into bone.
To determine how well the thyroid gland is functioning, doctors usually measure the levels of TSH, T4 and T3 in the blood.
Usually the TSH level is the best indicator of thyroid function. Because the hormone stimulates the thyroid gland, blood levels of TSH are high when the thyroid gland is underactive (and thus needs more stimulation) and low when the thyroid gland is overactive (and thus needs less stimulation). However, in the rare cases in which the pituitary gland is not functioning normally, the level of TSH does not accurately reflect thyroid gland function.
When doctors measure the levels of thyroid hormones T4 and T3 in the blood, they usually measure both the bound and free forms of each hormone (total T4 and total T3). However, if the level of thyroxine-binding globulin is abnormal, the total thyroid hormone levels can be misinterpreted, so doctors sometimes measure the level of the free hormones in the blood. The level of thyroxine-binding globulin is lower in people who have kidney disease or diseases that reduce protein synthesis by the liver or who take anabolic steroids. The level is higher in women who are pregnant or taking oral contraceptives or other forms of estrogen and in people in the early stages of hepatitis.
If a doctor feels one or more growths (nodules) in the thyroid gland, a scanning produce may be done. An ultrasound scan uses sound waves to measure the size of the gland and to determine whether the growth is solid or filled with fluid (cystic). In another type of thyroid scan, a small amount of radioactive iodine or technetium is injected into the bloodstream. The radioactive material concentrates in the thyroid gland and a device (gamma camera) that detects the radiation produces a picture of the thyroid gland that will show any physical abnormalities. Thyroid scanning can also help determine whether the functioning of a specific area of the thyroid is normal, overactive, or underactive compared with the rest of the gland.
Additional resting may be necessary in rare cases in which doctors cannot determine whether the problem lies in the thyroid or in the pituitary gland.
If cancer of the thyroid gland is suspected, doctors use a small needle to obtain a sample of thyroid tissue for study (a biopsy). When medullary thyroid cancer is suspected, blood levels of calcitonin are measured, because these cancers always secrete calcitonin.
Hyperthyroidism has several causes, including Graves’ disease, thyroiditis, inflammation rom toxic substances or radiation exposure, toxic thyroid nodules, and overstimulation due to an overactive pituitary gland.
Graves’ disease, the most common cause of hyperthyroidism, is an autoimmune disorder caused by an abnormal protein (antibody) in the blood that stimulated the thyroid to produce and secrete excess thyroid hormones into the blood. The cause of hyperthyroidism is often hereditary especially in women, and almost always leads to a diffusely enlarged thyroid.
Thyroiditis is inflammation of the thyroid gland. In subacute thyroiditis, silent lymphocytic thyroiditis, and, much less often, Hashimoto’s thyroiditis, hyperthyroidism occurs as stored hormones are released from the inflamed gland. Hypothyroidism usually follows because the levels of stored hormones are depleted. Finally, the gland usually returns to normal function.
Prognosis and Treatment
Treatment of hyperthyroidism depends on the cause. In most cases, the problem causing hyperthyroidism can be cured, or the symptoms can be eliminated or greatly reduced. If left untreated, however, hyperthyroidism places undue stress on the heart and many other organs.
Beta-blockers such as propranolol help control many of the symptoms of hyperthyroidism. These drugs slow a fast heart rate, reduce tremors, and control anxiety. Doctors therefore find beta-blockers particularly useful for people with extreme hyperthyroidism and for people with bothersome or dangerous symptoms that have not responded to other treatments. However, beta-blockers do not reduce excess thyroid hormone production. Therefore, other treatments are added to bring hormone production to normal levels.
Propylthiouracil and methimazole are the drugs most commonly used to treat hyperthyroidism; they work by decreasing the gland’s production of thyroid hormones. Each drug is taken by mouth, beginning with high doses that are later adjusted according to blood test results. These drugs can usually control thyroid function in 6 to 12 weeks. Larger doses of these drugs may work more quickly but increase the risk of side effects. Pregnant women who take propythiouracil or methimazole are closely monitored, because these drugs across the placenta and can induce goiter or hyperthyroidism in the fetus. Carbimazole, a drug that is widely used in Europe, is converted into methimazole in the body.
Iodine, given by mouth, is sometimes used to treat hyperthyroidism. It is reserved for those in whom rapid treatment is needed. It may also be used to control hyperthyroidism until the person can have surgery to remove the thyroid. It is not used long-term.