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Clinical Observation |

Raloxifene Causing Malabsorption of Levothyroxine

Elias S. Siraj, MD; Manjula K. Gupta, PhD; S. Sethu K. Reddy, MD
Arch Intern Med. 2003;163(11):1367-1370. doi:10.1001/archinte.163.11.1367.
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To our knowledge, raloxifene hydrochloride, a selective estrogen receptor modulator, has never been reported to interfere with absorption of levothyroxine. We describe a 79-year-old woman with chronic, treated primary hypothyroidism, presenting with increasing levothyroxine requirement while taking raloxifene at the same time as levothyroxine. For two 6- to 8-week periods, we separated the ingestion of raloxifene and levothyroxine by about 12 hours. In addition, we tested the absorption of 1.0 mg of levothyroxine sodium with and without the coadministration of 60 mg of raloxifene hydrochloride on 2 separate occasions by collecting serial blood samples for 6 hours. Hypothyroidism occurred in a reproducible fashion whenever levothyroxine and raloxifene were administered together and improved whenever they were taken separately. Combined administration of levothyroxine and raloxifene resulted in lower levels of serum thyroxine compared with administration of levothyroxine alone. By a yet unknown mechanism, raloxifene caused malabsorption of levothyroxine in our patient when coadministered.

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Figure 1. Course of patient with suspected levothyroxine sodium malabsorption secondary to raloxifene hydrochloride use. TSH indicates thyroid-stimulating hormone (normal value for TSH, 0.4-5.5 µU/mL).

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Figure 2. Serum thyroxine (T4) levels following oral administration of levothyroxine sodium with and without the coadministration of raloxifene hydrochloride. To convert T4 values to nanomoles per liter, multiply by 12.9.

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