An overactive thyroid causes abnormally high levels of thyroid hormones in the body. The thyroid produces two hormones, triiodothyronine (T3) and thyroxine (T4). These hormones control the activity level of cells throughout the body. An overactive thyroid--hyperthyroidism--increases metabolism causing symptoms such as hyperactivity, nervousness, hand tremor, excessive sweating, weight loss and digestive problems. Medicines that block thyroid hormone production and release can counteract an overactive thyroid.
Methimazole
Methimazole is an antithyroid medication. It inhibits the enzyme thyroid peroxidase, thereby blocking the production of the thyroid hormones. Because methimazole does not affect thyroid hormone stores, it usually takes several weeks of treatment for T3 and T4 to drop to normal levels. Once thyroid hormone levels normalize, a lower maintenance dose of the medication enables the thyroid to produce normal levels of T3 and T4 while blocking excessive production.
Propylthiouracil
Like methimazole, propylthiouracil is an antithyroid medication. By blocking the enzyme thyroid peroxidase, the drug decreases the production of thyroid hormones. Improvement in symptoms may take several weeks until the stores of previously produced thyroid hormones are depleted. A lower maintenance dose of the drug is typically used for 12 to 18 months once T3 and T4 drop to normal levels. Propylthiouracil is less potent than methimazole. Therefore, some doctors prefer propylthiouracil for mild to moderate hyperthyroidism, and reserve the use of methimazole for severe cases.
The American Association of Clinical Endocrinologists notes in "Medical Guidelines for Clinical Practice for the Evaluation and Treatment of Hyperthyroidism and Hypothyroidism" that successful treatment of hyperthyroidism requires taking antithyroid medication--propylthiouracil or methimazole-- consistently as prescribed.
Radioactive Iodine Therapy
Radioactive iodine, or RAI, may be used to treat hyperthyroidism. The thyroid gland actively takes up iodine from the blood stream because it is a necessary component of the thyroid hormones. When the thyroid gland absorbs RAI, the drug emits gamma and beta radiation, which kill thyroid cells over a period of weeks to months.
The development of an underactive thyroid gland, or hypothyroidism, is a risk of radioactive iodine therapy. A 2010 review article titled, "Current and Emerging Treatment Options for Graves' Hyperthyroidism," reports that approximately 3 percent of people successfully treated with RAI go on to develop an underactive thyroid gland each year. It also points out, however, that hypothyroidism is medically preferable to an overactive thyroid gland because the condition is more easily treated.
References
- American Association of Clinical Endocrinologists: Medical Guidelines for Clinical Practice for the Evaluation and Treatment of Hyperthyroidism and Hypothyroidism
- Endocrine Society Clinical Practice Guideline: Management of Thyroid Dysfunction during Pregnancy and Postpartum
- Merck Manual: Hyperthyroidism Treatment
- Mayo Clinic: Hyperthyroidism Treatments and Drugs
- EndocrineWeb: Treatment Options for Hyperthyroidism
