Is there an association between levothryoxine and breast cancer?

The short answer is that there is not sufficient evidence to draw any conclusion at this time. Patients who require levothyroxine should continue to take their medication as prescribed by their doctor. The risk of ultimately fatal myxedema due to severe untreated hypothyroidism is proven and real. The risk of cancer is speculative and not definitively proven.

When this question came up at the end of the lecture, my initial thought was that an association between levothryoxine and cancer is highly improbable. Oral levothyroxine essentially replaces your own endogenous thyroxine (T4). Treatment normalises T4 levels but should not cause supraphysiological levels of T4.  Even the route of absorption is largely physiological as endogenous T4 is recycled through enterohepatic circulation and is absorbed in the small intestine after being excreted in the bile. The only potentially “unnatural” exposure is as the levothryoxine tablet passes from the mouth to the stomach. In theory, there is no reason why taking levothyroxine should expose anyone to greater risk than having physiologically normal thyroid function would.
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