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TomHsiung
Wed 1st May '13, 11:09am
Recent months, I found some of docs in my hospital had prescribed both levothyroxine and methimazole, which is called "block and replace" approach, to patients with diagnosis of drug-induced hypothyroidism. Docs said the hypothyroidism was due to methimazole.

The are two approach of antithyroid agents after patients get euthyroid. The first is decreasing the dose of methimazole and maintain the euthyroid status by minimal dose of methimazole;the second is not decreasing the dose of methimazole and adding levothyroxine to maintain the euthyroid status, which is called "block and replace".

I looked up the antithyroid agent treatment approach in the guideline of hyperthyroidism published in 2011 by American Thyroid Association (ATA) and American Association of Clinical Endocrinologists (AACE). The guideline recommend avoiding the approach of "block and replace". The reason of this suggestion probably is due to the concern of the adverse effect of methimazole.

It is strange that docs in my hospital do not use both two approaches above. They decrease the dose of methimazole and add levothyroxine simultaneously.

So I contacted one of the doctor.

She told me that they had decreased the dose of methimazole to the lowest but these patients still remained hypothyroidism. Finally the had to add levothyroxine to make these patients euthyroid. My question is that is it possible that these patients were initially hyperthyroidism and after antithyroid agent therapy they becaome hypothyroidism even the dose of the agent was the lowest? I think it is not possible in clinical practice.

I assume that the dose of methimazole in these patients were not optimal so hypothyroidism happened. What do you think?