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Levothyroxine should be taken in a fasting state

Reuters Health • The Doctor's Channel Daily Newscast

NEW YORK (Reuters Health) – For hypothyroid patients taking levothyroxine, therapeutic levels are best maintained by taking the medication in a fasting state, at least an hour before breakfast, new research indicates.

Levothyroxine has a relatively narrow “toxic to therapeutic ratio,” the researchers note. Patients are usually advised to take it on an empty stomach, but how long they should wait between taking the medication and eating, and the optimal time of day for greatest absorption, were not known.

Dr. Jacqueline Jonklass and associates at Georgetown University Medical Center in Washington, DC, conducted a randomized crossover study involving three 8-week regimens. One regimen involved taking the drug after an overnight fast and at least 1 hour before breakfast; one involved taking it at bedtime and at least 2 hours after the last meal of the day; and the third involved taking it in a fed state, within 20 minutes of breakfast.

The primary outcome was the difference between the serum level of thyroid stimulating hormone (TSH) under fasting conditions and with each of the other two regimens. Blood for thyroid function tests was drawn at baseline and after each 8-week period at 8:00 in the morning.

Included were 65 patients treated with levothyroxine for at least 2 years. Patients taking medications or on diets known to interfere with levothyroxine absorption, or serious chronic illnesses, were excluded.

According to the report in the October Journal of Clinical Endocrinology and Metabolism, the mean TSH concentration after the overnight fast regiment was 1.06 mIU/L. By comparison, mean TSH levels were 2.19 mIU/L when the medication was taken at bedtime and 2.93 mIU/L when it was taken after breakfast.

“Nonfasting regimens of levothyroxine administration are associated with higher and more variable TSH concentrations,” the researchers said.

To avoid subclinical thyroid disease and to keep TSH concentrations within the narrowest target range, levothyroxine needs to be taken on an empty stomach, they emphasize.

“This admonition may be particularly important for patients who are pregnant, elderly, or have diagnoses of thyroid cancer, cardiac disease, or osteoporosis because specific TSH targets are of great importance in these populations.”

J Clin Endocrinol Metab 2009;94:3905-3912.