Synthetic thyroid preparations are taken by about 2 percent of the U.S. population and account for approximately 1 percent of prescriptions each year. Correct use often requires dosage titration or adjustments based on the patient's thyroid-stimulating hormone (TSH) level. Studies performed over the past 12 years indicate that excess exogenous thyroid hormone contributes to osteoporosis and cardiac problems, including left ventricular hypertrophy and atrial fibrillation. Watsky and Koeniger performed a retrospective study to determine the prevalence of inappropriate TSH suppression in patients receiving levothyroxine.

The authors reviewed the medical records of all patients who received a prescription for levothyroxine from a military base pharmacy in 1994 and in whom at least one TSH determination was obtained during the same year. TSH suppression was defined as a TSH level of less than 0.1 μU per mL. The records of all patients with a suppressed TSH level were further examined to determine diagnoses and duration of levothyroxine therapy. Suppressive therapy was considered appropriate in patients with thyroid cancer, nodular thyroid disease, goiter and central hypothyroidism (although the authors cite this approach as controversial). TSH suppression was considered inappropriate in patients who had suppressed TSH values and were being treated for primary hypothyroidism or were receiving thyroid hormone for questionable indications, such as chronic fatigue or obesity.

Out of 1,652 patients who had received a prescription for levothyroxine during 1994, 905 had at least one TSH value measured at the base laboratory. Of these 905 patients, 110 (12.2 percent) had a TSH level of less than 0.1 μU per mL on at least one occasion. Of these 110 patients, 34 (30.9 percent) had a diagnosis for which suppressive therapy might be appropriate. However, therapy was considered inappropriate in 63 (57.3 percent) of the 110 patients with TSH suppression. Also of note were 116 patients (12.8 percent) with TSH values higher than 5.66 μU per mL, suggesting undertreatment with levothyroxine.

Overt hyperthyroidism, defined as a free thyroxine (T4) level above the upper limit of normal, was present in 21 (2.3 percent) of the 905 patients. Three of these patients were in the group in whom TSH suppression was considered appropriate, and 18 were in the group in whom TSH suppression was considered inappropriate.

The authors conclude that many patients appear to be inappropriately treated with levothyroxine. They also note that TSH suppression for thyroid cancer and nodular goiter is controversial and not fully supported by the literature. If the cutoff for TSH suppression were a level of 0.4 μU per mL, as suggested by some authors, the degree of overtreatment in this study group would have been much greater. Women appear to be at greater risk for inappropriate treatment with levothyroxine. The authors encourage physicians to educate patients about the potential toxicity associated with thyroid suppression and to prescribe levothyroxine only for patients who are most likely to benefit from this therapy.