Raloxifene May Block L-Thyroxine Absorption

OB/GYN News, Oct 1, 2001 by Betsy Bates

SAN ANTONIO - Patients taking raloxifene and L-thyroxine should be advised not to take the two drugs at the same time, Dr. Elias S. Siraj said at the annual meeting of the American Association of Clinical Endocrinologists.

Dr. Siraj of the Cleveland Clinic reported the case of a 79-year-old woman who required increasing doses of L-thyroxine to manage her symptoms of hypothyroidism after initiating raloxifene therapy. Her case and a subsequent study documented malabsorption of L-throuxine when the drug was taken at the same time as raloxifene. Taking the two drugs at different times of the day did not impair L-thyroxine absorption.

The patient's primary hypothyroidism had been well managed for many years at a dosage of 0.15 mg/day of L-thyroxine. She had undergone a subtotal thyroidectomy 30 years before for benign thyroid nodules.

She had been prescribed 60 mg/day of raloxifene, marketed as Evista, for osteopenia. She took the raloxifene early in the morning, at the same time that she took her daily dose of L-thyroxine.

At noon she took sertraline, hydrochlorothiazide, ferrous sulfate, and a multivitamin. She received monthly [B.sub.12] injections. Ferrous sulfate is known to inhibit absorption of L-thyroxine, as are several other medications including sucralfate, cholestyramine, colestipol, and aluminum-containing antacids.

Also of potential relevance, the patient had a history of pernicious anemia but had reported no gastrointestinal symptoms, Dr. Siraj noted.

Within 2 months of initiating raloxifene, the patient reported symptoms of hypothyroidism. Her TSH level was measured at 14.5 [micro]U/mL. The woman's dosage of L-thyroxine had increased to 0.3 mg/day at 7 months after beginning raloxifene therapy. A study was undertaken and demonstrated that the problem was related to taking the two medications at the same time.

When raloxifene and L-thyroxine were taken together for 8 weeks, the patient's TSH level was 9.36 [micro]U/mL and the [T.sub.4] was 10.2 [micro]g/dL. When taken separately, the investigators reassessed the thyroid levels, and the TSH was 0.63 [micro]U/mL and the [T.sub.4] was 13.3 [micro]g/dL.

The drugs were again given together. The TSH rose to 13 [micro]U/mL, and the [T.sub.4] was 9.3 [micro]g/dL. When the drugs were administered separately, the TSH was 2.2 [micro]U/mL and the [T.sub.4] was 12 [micro]g/dL.

Two 6-hour absorption tests were then performed in which 0.1 mg of L-thyroxine was administered alone and then with 60 mg of raloxifene.

Levels of [T.sub.4] looked similar initially, but by 60 minutes the [T.sub.4] was 23.7 [micro]g/dL without raloxifene and 16.2 [Micro]g/dL with raloxifene. At 90 minutes, the levels were 29.7 [micro]g/dL and 18.8 [micro]g/dL, respectively; at 120 minutes, the levels were 28.5 [micro]g/dL and 18.8 [micro]g/dL.