Short-term Withdrawal of Levothyroxine, Induced Increase of Thyroid-stimulating Hormone and an Increase Ratio of Triiodothyronine to Thyroxine.

Abstract

Objective: Treatment with levothyroxine in primary hypothyroid patients does not always provide complete regression of associated symptoms despite normalised TSH levels. Several sources report ratios of triiodothyronine (T3) to thyroxine (T4) are diminished in hypothyroid patients following a daily levothyroxine regimen. It is known that thyroid-stimulating hormone (TSH) increases de-iodination of T4 to T3. We hypothesise that a raise in TSH levels caused by a temporary withdrawal of oral levothyroxine will be followed by an increased conversion of T4 to T3. Methods: Thirteen patients treated with monotherapy of levothyroxine were included in our pilot study. Treatment was temporarily discontinued for one week in which TSH, free T3 (fT3) and free T4 (fT4) were monitored. TSH and fT3 to fT4 ratios were compared with baseline values. Results: Statistically significant elevations in TSH and fT3 plasma levels relative to fT4 were demonstrated in all patients after withdrawal of levothyroxine. Conclusion: Both TSH and fT3 to fT4 ratios rose following temporary discontinuation of levothyroxine. The effect on symptoms and quality of life is not evaluated in this pilot study. Our results warrant further investigation into whether or not longer dosing intervals would demonstrate commensurate hormone elevations that better reflects the hormonal ratios in healthy subjects and if this also has an effect on quality of life scores.