SUN-404 - Severe Hypothyroidism Triggered by Cholestyramine - Levothyroxine Interaction.

Introduction
Hypothyroidism is a condition characterized by insufficient production of thyroid hormone due to dysfunction of the thyroid gland, affecting an estimated 30 million adults in the United States. Levothyroxine (LT4) remains the mainstay of therapy. However, LT4 absorption can be significantly affected by concomitant medications, leading to suboptimal thyroid hormone replacement and clinical consequences. This case highlights a severe drug interaction between cholestyramine and levothyroxine, resulting in profound hypothyroidism.

Case report
A 54-year-old woman with a history of toxic nodular goiter underwent radioactive iodine ablation several years ago, subsequently developed primary hypothyroidism. She was started on levothyroxine 150 mcg daily, which led to normalization of her thyroid function and complete resolution of hypothyroid symptoms. Her medication regimen remained stable for several years. The patient presented later with worsening fatigue, lethargy, and generalized weakness. She denied missing any doses of her medication. On examination, her vital signs were stable with no evidence of goiter. Laboratory evaluation revealed a markedly elevated TSH (>150 mIU/L), low free T4 (0.7 ng/dL), and low total T3 (1.5 ng/mL), consistent with severe hypothyroidism. A thorough review of her medications revealed that cholestyramine had been initiated several weeks prior to onset of her symptoms. Recognizing the potential for cholestyramine to interfere with LT4 absorption, cholestyramine was discontinued. Four weeks later, her symptoms resolved, and repeat thyroid function tests showed normalization of TSH and free T4 levels.

Discussion
Cholestyramine is a bile acid sequestrant commonly used in the management of hyperlipidemia and cholestatic conditions. Cholestyramine can bind to the thyroid hormones in the gastrointestinal tract, thereby reducing the absorption and bioavailability of oral levothyroxine. This interaction can result in significant hypothyroidism, even in patients with stable on replacement therapy. Other commonly encountered medications that can impair levothyroxine absorption include calcium carbonate, antacids, iron supplements, and sevelamer.
To minimize the risk of drug interactions, it is recommended that levothyroxine be administered on an empty stomach, and that any interfering medications be taken at least 4–6 hours apart from Levothyroxine. Hypothyroid patients receiving cholestyramine should be monitored periodically for evidence of hypothyroidism.

Conclusion
This case underscores the importance of recognizing drug interactions as a cause of refractory or recurrent hypothyroidism. Patient education regarding the timing of levothyroxine and potential interactions is critical to ensure effective management of hypothyroidism.

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