Thyroid diseases

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The thyroid occupies a low position in the collective consciousness, yet this small gland influences virtually every cell in the body. The hormones it secretes into the bloodstream play a vital role in regulating our basal metabolism-the rate at which we convert food and oxygen to energy.

How the thyroid works
In some respects the thyroid is like a small factory. It is the site where iodide-the byproduct of the iodine we absorb in food-enters from the bloodstream, is converted back into its original form, and then is welded to proteins to form the hormones triiodothyronine (T3) and thyroxine (T4). T3 enters the cells directly and activates the genes that direct the production of certain proteins. T4 circulates in the blood until T3 is needed; then it is taken into the cells and converted to T3.

The thyroid's output is determined by the hypothalamus, a regulatory region of the brain. The hypothalamus sends thyrotropin-releasing hormone (TRH) to the pituitary-a peanut-sized gland at the base of the brain-which is entrusted with maintaining thyroid-hormone production at genetically determined levels. When the pituitary senses that the supply of thyroid hormones threatens to drop below those levels, it emits thyroid-stimulating hormone (TSH) to trigger the production and release of T3 and T4.

Because the thyroid hormones influence so many types of cells, it's not surprising that when something goes awry we may experience an array of symptoms that seem unrelated. Changes in weight, bowel habits, heart rate, hair growth, temperature tolerance, and menstrual cycles-when they occur together-can signal a thyroid condition.

Our susceptibility to thyroid disease is largely determined by the interaction of our genetic makeup, age, and gender. Women, particularly those with a family history of thyroid disease, are much more likely to have thyroid trouble than men. Fortunately, although most thyroid conditions can't be prevented, they respond well to treatment.

When hormone levels are too low
Hypothyroidism, too low a level of the hormones, is the most common thyroid disease in this country, with more than 50% of cases occurring in families where thyroid disease is present. Although as many as one woman in 10 develops hypothyroidism, its symptoms appear so gradually that many are unaware that they have it.

As thyroid hormone levels fall, the basal metabolic rate drops, resulting in a decreased heart rate, an increased sensitivity to cold, sluggishness, constipation, a slowdown in hair and nail growth, and a weight gain that is limited to 10-15 pounds. (Contrary to widespread misconception, hypothyroidism is not responsible for obesity.) Because blood hormone levels are consistently low, the pituitary sends a steady stream of TSH to the thyroid in an attempt to encourage production. This constant stimulation sometimes causes it to enlarge, creating a goiter.

Hypothyroidism may occur in people who take lithium or certain asthma medications, or who have undergone radiation therapy for certain cancers, such as Hodgkin's disease or throat cancer. Iodine deficiency, once a common cause of hypothyroidism, is no longer a problem in this country; most of us get more than enough iodine from salt, bread, milk, and certain prescription and over-the-counter medications.

Hashimoto's thyroiditis is among the most common causes of hypothyroidism in women over 50. It is an inherited condition in which the body produces antibodies against its own thyroid tissue, eventually damaging the gland so much that it can no longer produce enough T3 and T4.

The most accurate test for hypothyroidism measures levels of TSH in the blood. A high TSH level indicates that the pituitary is signalling the thyroid to produce more hormone in an attempt to compensate for too little T3 and T4.

Regardless of the form of hypothyroidism, the condition can be successfully treated with tablets containing thyroxine, or T4, which can be regulated to maintain adequate levels of circulating hormone. Blood levels of the hormones should be checked periodically and the dosage adjusted when necessary, because needs may vary with age, body weight, and use of certain medications. Most patients need to take the thyroxine for the rest of their lives.

When levels are too high
As might be expected, the symptoms of hyperthyroidism are the opposite of those that characterize hypothyroidism-a modest weight loss despite a normal appetite, bouts of mild diarrhea or frequent bowel movements, heat sensitivity, trembling, increased heart rate, and emotional changes. Women with hyperthyroidism have lighter periods and a higher rate of infertility and miscarriage.

There are several conditions that can cause hyperthyroidism, but about half of all cases are the result of Graves' disease, which, like Hashimoto's thyroiditis, is hereditary. It is seven to nine times more common in women than in men. It is also an autoimmune disease, but instead of destroying the thyroid tissue, the antibodies in patients with Graves' disease mimic the effects of TSH, triggering thyroid enlargement.

Like those with other forms of hyperthyroidism, people with Graves' disease often take on a "wide-eyed" look, caused by a retraction of the upper lids. Less commonly, the immune reaction responsible for Graves' disease creates swelling in the muscles of the eyes, causing them to bulge, and produces raised, plaque-like areas on the skin of the legs.

In those with a family history of Graves' disease, the condition is most likely to strike between the ages of 20 and 40, sometimes in response to a hormonal change, such as pregnancy. It may also occur after severe emotional stress.

Evaluation for Graves' disease includes a blood test for low levels of TSH and often an iodine uptake test, in which the patient swallows a solution containing radioactive iodine. The physician then uses a scanning devise to measure the amount of iodine that has been absorbed by the thyroid; an elevated level further confirms that the gland is overactive.

Younger patients with mild Graves' disease can be successfully treated with drugs, such as propylthiouracil (PTU) or methimazole, for a period of 6 to 12 months. These drugs make it impossible for the thyroid to use iodine and thus block hormone production. Because antithyroid drugs have several undesirable side effects, many doctors now treat patients initially with radioactive iodine, which destroys thyroid tissue. The dose used to treat hyperthyroidism is much larger than that used to diagnose it, but it passes out of the body through the urine in 48 hours.

Regardless of the method of treatment, high levels of circulating thyroid hormone may remain in the blood for several weeks, so another drug called a beta-blocker is often used to counteract the thyroid hormone's effects on the heart and vascular system. A majority of those treated for Graves' disease will eventually have low thyroid function and require thyroid pills.

Other forms of hyperthyroidism
A single lump, or nodule, which often develops in the thyroids of people who received low-dose radiation in childhood, can sometimes secrete high levels of hormone. When the nodule's production reaches an excessive amount, symptoms of hyperthyroidism begin.

When there are more than one nodule, the resulting condition is called toxic multinodular goiter. It usually occurs in people over age 65, and is responsible for about 25% cases of hyperactive thyroid.

Although beta-blockers and antithyroid drugs can alleviate the symptoms caused by thyroid nodules, the only permanent cure is radioactive iodine to destroy the active thyroid tissue. It also eventually necessitates thyroid supplements.

Radioactive iodine therapy has been so successful that thyroidectomy, or removal of the gland, is now rarely used to treat hyperthyroidism. Surgery is usually reserved for patients with goiters that interfere with breathing or swallowing or are unsightly.

Subacute thyroiditis is a less common form of hyperthyroidism caused by a viral infection. In addition to flu-like symptoms, the patient experiences an inflammation of the thyroid, which allows excess hormone to leak out into the bloodstream. Although the effects of this form of hyperthyroidism can also be countered with beta-blockers, the symptoms and the underlying infection eventually clear up without treatment. A painless form of thyroiditis occasionally occurs in women who have recently given birth.

Thyroid cancer
Rarely, a lump in the thyroid will have more serious implications. Hoarseness, difficulty swallowing, and a rapid enlargement of a nodule that is not accompanied by symptoms of hypo- or hyperthyroidism may be signs of thyroid cancer. Fortunately, this form of cancer is rare, occurring in 25 people per million, and usually curable. It is most common in those who have received radiation to the head or neck in childhood or who have a family history of thyroid cancer.

When doctors evaluate a thyroid lump, they perform blood tests for hormone levels and autoantibodies to rule out other conditions. A thyroid scan can determine if the nodules fail to take up radioactive iodine and are therefore "cold" or nonfunctional, which raises the suspicion of cancer. Ultrasound or needle biopsy can indicate whether a cold nodule is a benign fluid-filled cyst or a potentially malignant one. Treatment for thyroid cancer entails removal of all or part of the thyroid, and if cancer has metastasized, therapy with radioactive iodine to destroy the remaining malignant cells is recommended.

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