Left untreated, underactive thyroid (hypothyroidism) caused by Hashimoto's disease can lead to a number of health problems:

• Goiter. Constant stimulation of your thyroid to release more hormones may cause the gland to become enlarged, a condition known as goiter. Hypothyroidism is one of the most common causes of goiter. Although generally not uncomfortable, a large goiter can affect your appearance and may interfere with swallowing or breathing.
• Heart problems. Hashimoto's disease also may be associated with an increased risk of heart disease, primarily because high levels of low-density lipoprotein (LDL) cholesterol — the "bad" cholesterol — can occur in people with an underactive thyroid (hypothyroidism). Hypothyroidism caused by Hashimoto's disease also can lead to an enlarged heart and, in rare cases, heart failure.
• Mental health issues. Depression may occur early in Hashimoto's disease and may become more severe over time. Hashimoto's disease can also cause sexual desire (libido) to decrease in both men and women and can lead to slowed mental functioning.
• Myxedema. This rare, life-threatening condition develops due to long-term hypothyroidism as a result of untreated Hashimoto's disease. Its signs and symptoms include intense cold intolerance and drowsiness followed by profound lethargy and unconsciousness. A myxedema coma may be triggered by sedatives, infection or other stress on your body. Myxedema requires immediate emergency medical treatment.
• Birth defects. Babies born to women with untreated Hashimoto's disease may have a higher risk of birth defects than do babies born to healthy mothers. Doctors have long known that these children are more prone to intellectual and developmental problems. There may be a link between hypothyroid pregnancies and birth defects such as cleft palate. A connection also exists between hypothyroid pregnancies and heart, brain and kidney problems in infants. However, if any of these conditions are diagnosed within the first few months of a baby's life, chances of normal development are excellent.

Treatment for Hashimoto's disease may include observation and use of medications. If there's no evidence of hormone deficiency and your thyroid is functioning normally, your doctor may suggest a wait-and-see approach.

Synthetic hormones
If Hashimoto's disease causes thyroid hormone deficiency, you may need replacement therapy with thyroid hormone. This usually involves daily use of the synthetic thyroid hormone levothyroxine (Levothroid, Synthroid). The oral medication restores adequate hormone levels, returning your body to its normal functioning.

Soon after starting treatment, you'll notice that you're feeling less fatigued. The medication also gradually lowers cholesterol levels elevated by the disease and may reverse any weight gain. Treatment with levothyroxine is usually lifelong, but because the dosage you need may change, your doctor is likely to check your TSH level every year or so.

Monitoring the dosage
To determine the right dosage of levothyroxine initially, your doctor generally checks your level of TSH after two to three months. Excessive amounts of the hormone can accelerate bone loss, which may make osteoporosis worse or add to your risk of this disease.

If you have coronary artery disease or severe hypothyroidism, your doctor may start treatment with a smaller amount of medication and gradually increase the dosage. Progressive hormone replacement allows your heart to adjust to the increase in metabolism.

Levothyroxine causes virtually no side effects when used in the appropriate dose and is relatively inexpensive. If you change brands, let your doctor know to ensure you're still receiving the right dosage. Also, don't skip doses or stop taking the drug because you're feeling better. If you do, signs and symptoms will gradually return.

Effects of other substances
Certain medications, supplements and even some foods may affect your ability to absorb levothyroxine. Talk to your doctor if you eat large amounts of soy products or a high-fiber diet, or if you take any of the following:

• Iron supplements
• Cholestyramine (Questran), a medication used to lower blood cholesterol levels
• Aluminum hydroxide, which is found in some antacids
• Sodium polystyrene sulfonate (Kayexalate), used to prevent high blood potassium levels
• Sucralfate, an ulcer medication

Although most doctors recommend synthetic thyroxine (levothyroxine), natural extracts containing thyroid hormone derived from the thyroid glands of pigs are available. These products — Armour Thyroid Hormone and Bio-Throid, for example — contain both thyroxine and triiodothyronine. Synthetic thyroid medications contain thyroxine only, and the triiodothyronine your body needs is derived from the thyroxine.

Extracts are available by prescription only. They're different from the glandular concentrates sold in natural foods stores. Glandulars are dried concentrates of glands derived from animals. These products aren't regulated by the Food and Drug Administration, and their potency isn't guaranteed. What's more, using them raises concerns about exposure to bovine spongiform encephalopathy, a progressive neurological disorder of cattle. Some, but not all, glandular products are derived from range-fed cattle from New Zealand or Argentina, which are more likely to be disease-free.

Talk with your doctor before taking any dietary or herbal supplement.