Raloxifene decreased the intestinal absorption
of thyroxine in a patient with hypothyroidism
(November 2003)
The background of the study. Several
drugs and high doses of iron and calcium decrease the intestinal
absorption of thyroxine (T4) in patients with hypothyroidism. This
case report describes a patient in whom raloxifene, a selective
estrogen-receptor modulator, caused a decrease in the absorption
of T4.
Case report. A 79-year-old woman
with hypothyroidism treated with 0.15 mg of T4 daily was well and
had normal serum thyrotropin (TSH) concentrations for several years.
She was found to have low bone density, and was treated with 60
mg of raloxifene daily, which she took with the T4 early in the
morning. She began to have symptoms of hypothyroidism, and a month
later her serum TSH concentration was high (14.5 mU/L). The dose
of T4 was increased to 0.2 mg daily, but she continued to have symptoms;
four months later her serum TSH concentration was 21.4 mU/L. The
dose of T4 was raised to 0.3 mg daily; two months later her serum
TSH concentration was 9.4 mU/L. She had no symptoms of malabsorption
of food.
She was advised to take T4 and raloxifene at different times; two
months later her serum TSH concentration was 0.6 mU/L. She then
took the T4 and raloxifene at the same time, and a month later her
serum TSH concentration was 13.0 mU/L; it fell to 2.2 mU/L when
she again took the T4 and raloxifene separately.
On two occasions she was given 1 mg of T4, once without and once
with 60 mg of raloxifene, and serum T4 was measured at frequent
intervals for six hours. There was a smaller rise in serum T4 concentration
when she took the two medications together.
The conclusions of the study. Raloxifene
inhibits the intestinal absorption of T4.
The original article. Siraj ES,
Gupta MK, Reddy SS. Raloxifene causing malabsorption of levothyroxine.
Arch Intern Med 2003;163:1367-70.
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