For a variety of reasons, it is becoming clear that a large portion of the U.S. population is hypothyroid. Most are taking some version of T4 (levothyroxine). The question is, how much should one take, and how should one gauge the dosage?

For years there has been concern that excessive doses of thyroid hormone could cause osteoporosis or cardiovascular problems such as atrial fibrillation. Now a new study, published in the Journal of Clinical Endocrinology and Metabolism in January 2010 provides some very surprising answers.

Following 17,684 people taking T4 between 1993 and 2001 these researchers found that the risk of cardiovascular disease, dysrhythmias, and fractures was no different from the normal population when the TSH (thyroid stimulating hormone) was partially suppressed below the normal range, but not completely suppressed. A TSH of 0.04 seemed to be optimal. The risk of cardiovascular disease, dysrhythmias, and fractures was higher when the TSH was either higher than that, or lower than that!

Thus "it may be safe for patients to be on a dose of T4 that results in a low serum TSH concentration, as long as it is not suppressed at less than 0.03 mU/liter." This is associated with the best outcomes in these studies.