Low thyroid


Many of us accept slowing down as an inevitable consequence of aging. Women in midlife often write off a growing sluggishness, weight gain, dry skin, cold feet, and a sense of depression as little more than a sign that we are getting older. However, these symptoms are more likely to be the result of decreasing levels of thyroid hormone than of increasing years. Moreover, they are not uncommon; one in 10 women over age 50 has hypothyroidism, or inadequate thyroid hormone levels.

The function of thyroid hormone is to regulate the rate of bodily processes. There are two primary types of thyroid hormone, triiodothyronine (T3) and thyroxine (T4), which are made from the iodine we ingest in foods such as salt, seafood, bread, and milk. Like other hormones, T3 and T4 are released into the blood and borne to distant parts of the body, including the brain, heart, liver, kidneys, bone, and skin. T3 attaches directly to receptors in the cell nucleus; T4 is converted to T3 inside the cell. The hormone activates genes for proteins that regulate body functions. Both thyroid hormones are released when a drop in circulating levels is registered by the hypothalamus -- the regulatory region of the brain. The hypothalamus sends a chemical message to the pituitary gland at the base of the brain, which in turn sends thyroid-stimulating hormone (TSH) to the thyroid to trigger the release of T3 and T4.

The symptoms of hypothyroidism appear so gradually and are so diverse that the disease often goes undetected. As thyroid hormone levels fall, the pituitary sends a steady stream of TSH to the thyroid to trigger the release of more hormone. The constant stimulation may cause the thyroid to enlarge, creating a goiter.

Declining thyroid hormone levels also have consequences throughout the body. The basal metabolic rate falls and, as a result, heart rate slows, and body temperature drops. Other problems, such as constipation, thinning hair, dry skin, and muscle cramps, may develop. Total cholesterol and LDL (bad) cholesterol levels can rise. Weight may slowly mount, but the gain rarely exceeds 10 or 15-pounds, most of which is due to fluid accumulation in the tissues. Difficulty with problem solving and memory -- as well as depression -- are also common.

Hashimoto's thyroiditis, a hereditary condition in which the immune system produces antibodies against its own thyroid tissue, is the most common cause of hypothyroidism. It occasionally lasts only a few months, and thyroid function returns to normal shortly thereafter. However, in most cases, it is a chronic condition, proceeding indefinitely. Hashimoto's disease occasionally occurs following childbirth even in women who once had hyperactive thyroids. This form, called postpartum thyroiditis, is thought to be due to the re-activation of the immune system, which is depressed during pregnancy to avoid miscarriage. Most women recover completely from postpartum thyroiditis, but about 25% never regain adequate thyroid function. Whatever the variant of Hashimoto's disease, the inflammation and scarring that result may damage the thyroid so extensively that the gland can no longer make adequate supplies of thyroid hormones. Viral infections of the thyroid can have a similar effect.

Hypothyroidism is often the result of radiation therapy for conditions such as Hodgkin's disease, adenoids, or throat cancers. In such cases, radiation to the neck or throat may damage the thyroid.

Drug treatment may also be responsible. Lithium, which is used for depression and other psychiatric disorders, can cause hypothyroidism in some people. However, medication to treat overactive thyroid is the most common medical cause. It may overcorrect the problem, converting an overactive thyroid into an underactive one.

Iodine, paradoxically, can also be a cause of hypothyroidism in people who are very sensitive to it. In such cases, they may unwittingly consume too much of it, often through eating kelp or seaweed, which are iodine-rich. Amiodarone, a drug taken for heart rhythm disturbances, also contains relatively large amounts of iodine.

The presence of symptoms listed above, especially when the thyroid is enlarged, is seen as an indication of hypothyroidism. The condition is confirmed by a blood test to determine whether levels of thyroid-stimulating hormone (TSH) are above 10 mU/L -- an indication that thyroid hormone levels are low. Guidelines published by the American College of Physicians in 1998 suggest TSH screening for women age 50 and older who have any such symptoms.

Thyroid medication is designed to turn up the metabolic rheostat. Originally, patients with hypothyroidism were treated with extracts containing both T3 and T4 from animal thyroids. However, the potency of the hormones contained was inconsistent. Occasionally, a patient reported heart palpitations or dizziness induced by the faster-acting T3. Today, hypothyroidism is customarily treated with a synthetic hormone that is chemically identical to T4 and is converted to T3 in most tissues. Because a person's needs for thyroid hormone can vary with age, weight, and other factors, brand-name (Synthroid, Levothroid, Levoxyl, Elthoxin) and generic T4 are formulated in several doses to allow adjustment. Doctors may need to check blood levels of the hormone to determine if the dose is too high, too low, or right on target. Excessive doses of hormone can put stress on the heart and increase the risk of osteoporosis by accelerating bone turn-over.

A recent Lithuanian report indicated that some people with hypothyroidism do better on a combination of synthetic T4 and T3. The researchers explained that T3 was added because it acts more rapidly than T4 and is taken in by some cells that do not admit T4. In that study, 20 of the 33 patients (31 of whom were women) reported that they felt somewhat better when they were taking the combination treatment than when they were taking T4 alone; they also had higher scores on standardized tests that assess memory and cognitive function. The combination might eventually prove to be helpful for the small percentage of patients who feel somewhat under par, even when T4 treatment brings hormone levels into the normal range. However, because the combination that is now available contains a proportionately higher dose of T3 than that found in the body, many experts believe it could cause heart problems in older patients. It's probably wise to wait until newer formulations with lower doses of T3 become available.

Thyroid medication can interact with other drugs. Cholestyramine and colestipol, which are used to treat high cholesterol levels; aluminum hydroxide, an antacid; Carafate, which treats stomach ulcers; and iron, taken for anemia, may prevent the absorption of thyroid hormone. Thus, they should be taken several hours after thyroid hormone.

In the past, patients were treated for hypothyroidism only after they developed symptoms. However, because routine blood tests often include TSH values, women who have high TSH but no symptoms, may be diagnosed with hypothyroidism. There is no clear consensus on whether these women should be treated to bring TSH down to normal levels or whether treatment should be postponed until they develop symptoms. The decision may call for a consideration of other factors, such as cholesterol levels.

Because thyroid damage that is discovered later in life is usually permanent, treatment for hypothyroidism can be a lifelong proposition. Although it's important to take the medication daily to maintain consistent levels of circulating hormones, forgetting to take a pill now and then usually doesn't have noticeable effects, because T4 remains in the blood for weeks after the last dose. In fact, people who discontinue medication may be deceived into thinking that their condition has reversed because they can continue to feel well for months. However, symptoms invariably return, albeit often as gradually as they originally occurred, as do the consequences of low thyroid -- including poor circulation and elevated LDL levels.

Thyroid foundation of America, Ruth Sleeper Hall (RSL) 350, 40 Parkman Street, Boston, MA 02114; www.tsh.org

American Association of Clinical Endocrinologists, 1000 Riverside Avenue, Suit 205, Jacksonville, FL 32204, (904) 353-7878; www.aace.com

American Foundation of Thyroid Patients, P.O. Box 820195, Houston, TX 77282-0195; (281) 496-4460 or (888) 996-4460; www.thyrooidfoundation.org

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