Hypothyroidism - Causes

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After Treatment of Hyperthyroidism

Up to half or more of patients who receive radioactive iodine treatments for an overactive thyroid develop permanent hypothyroidism within a year of therapy. This is the standard treatment for Graves' disease, which is the most common form of hyperthyroidism, a condition caused by excessive secretion of thyroid hormones.

By the end of 5 years, about 65% of treated patients develop hypothyroidism. Such patients need to take thyroid hormones for the rest of their lives. Other forms of treatment for overactive thyroid glands using either antithyroid drugs or surgery may also result in hypothyroidism.

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Iodine Abnormalities

Too much or too little iodine can cause hypothyroidism. If there is a deficiency of iodine, the body cannot manufacture thyroxine. About 200 million people around the world have hypothyroidism because of insufficient iodine in their diets. Too much iodine is a signal to inhibit the conversion process of thyroxine to T3. The end result in both cases is inadequate production of thyroid hormones. Some evidence suggests that excess iodine may trigger the process leading to Hashimoto's thyroiditis.

Thyroid Surgery

Patients who have complete removal (total thyroidectomy) of the thyroid gland to treat thyroid cancer need lifetime treatment with thyroid hormone. Removing one of the two lobes of the thyroid gland (hemithyroidectomy), usually because of benign growths on the thyroid gland, rarely produces hypothyroidism. The remaining thyroid lobe will generally grow so that it can produce sufficient amounts of thyroid hormone for normal function. Many doctors recommend thyroid hormone treatment, however, to prevent the formation of additional nodules.

Click the icon to see an illustrated series detailing thyroid removal.

Patients with Graves' disease who have surgery to remove most of both thyroid lobes (subtotal thyroidectomy) may develop hypothyroidism. It is important to find an experienced surgeon for this procedure and to have the thyroid checked at 6- or 12-month intervals.