Feasibility of reducing L-thyroxine dose in patients with a suppressed serum TSH.

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A total of 748 patients on L-thyroxine with a suppressed serum TSH were requested to reduce their dose and this was achieved in 601 patients. Thyroxine dosage was reduced by 25 or 50 micrograms of L-thyroxine and patients were reviewed six months later. Of all 601 patients, 54.4% remained with a suppressed serum TSH despite dose reduction and in 5.8% an elevated serum TSH resulted. 25 micrograms reductions and 50 micrograms reductions were equally likely to result in an detectable but non-elevated serum TSH (42.8% vs 34.1% ns) but 25 micrograms reductions were less likely to result in an elevated serum TSH (3.8% vs 10.0% p < 0.01). Only 7/601 patients in the study (1.2%) appeared to require a dose of over 150 micrograms. If dose reduction is thought to be necessary for patients with a suppressed serum TSH, we would recommend 50 micrograms reductions if the original dose is 200 micrograms or more, and 25 micrograms reductions if the original dose is 175 micrograms or less.