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The absorption of thyroid extract, levothyroxine and tri-iodothyronine from the gut is reduced by the concurrent use of cholestyramine. Separate the dosages by 4-5 h.

Clinical evidence

Prompted by the observation of a hypothyroidic patient under treatment with thyroxine whose basal metabolic rate fell when given cholestyramine, a further study was made on two similar patients taking 60 mg thyroid extract or 100 ng levothyroxine sodium daily, and on five normal subjects. When they were given 4 g cholestyramine four times daily their absorption of thyroxine was reduced and the amount remaining in the faeces was roughly doubled. One of the patients showed a worsening of her hypothyroidism. Separating the dosages by 4-5 h reduced the interaction to a minimum.

Mechanism

Cholestyramine binds to thyroxine m the gut, thereby reducings its absorption. Since thyroxine probably also takes part in the enterohepatic shunt (after absorption it is resecreted in the bile), continued contact with the cholestyramine is possible.

Importance and management

An established interaction (although the documentation is very limited) and of clinical importance. In vitro tests show that tri-iodothyronine interacts similarly. The effects can be minimized by separating the dosages by 4-5 h, even so the outcome should be monitored so that any necessary thyroid hormone dosage adjustments can be made.

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