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Context: The substitution of liothyronine (L-T3) for levothyroxine (L-T4) is commonly employed during thyroid hormone (TH) withdrawal in preparation for diagnostic and therapeutic interventions on thyroid cancer patients. Presently, only limited data are available on the L-T3 for L-T4 therapeutic substitution.

Objective: To characterize the pharmcodynamic equivalence of L-T3 and L-T4.

Design: Randomized, double-blind, cross-over intervention study.

Setting: NIH clinical center.

Patients: Ten thyroidectomized patients.

Interventions: Study participants were treated with L-T3 or L-T4 with a target TSH >= 0[middle dot]5 <= 1[middle dot]5 mU/l for at least 30 days before undergoing inpatient testing. Following testing, subjects crossed-over according to the same scheme.

Main outcome measures: Area under the serum concentration-time curve of TSH from 0 to 60 min (AUC0-60) and peak TSH serum concentration (Cmax) following thyrotropin-releasing hormone (TRH) stimulation test, total L-T4 and L-T3 dose (mcg/kg), and L-T4/L-T3 ratio.

Results: No difference was observed for time 0 TSH values between L-T3 and L-T4 replacement phases (1[middle dot]48 /- 0[middle dot]77 vs. 1[middle dot]21 /- 0[middle dot]62 mU/l, P = 0[middle dot]293) at average daily doses of 40[middle dot]3 /- 11[middle dot]3 mcg L-T3 and 115[middle dot]2 /- 38[middle dot]5 mcg L-T4, L-T3: L-T4 ratio 0[middle dot]36 /- 0[middle dot]06. TRH stimulation test resulted in similar L-T3 vs. L-T4 TSH responses with AUC0-60 of 326[middle dot]1 (95% CI 232[middle dot]6-457[middle dot]1) and 247[middle dot]1 (95% CI 153[middle dot]8-397[middle dot]1) mU* min/l (P = 0[middle dot]285); and Cmax of 6[middle dot]83 (95% CI 4[middle dot]88-9[middle dot]55) and 5[middle dot]23 (95% CI 3[middle dot]31-8[middle dot]3) mU/l (P = 0[middle dot]383).

Conclusions: This is the first study addressing the equivalency between L-T3 and L-T4 therapy measured by baseline and TRH-stimulated TSH. The therapeutic substitution of L-T3 for L-T4 was achieved at approximately 1:3 ratio.

Copyright (C) 2010 Blackwell Publishing Ltd.