T3 therapy may be substituted for T4 therapy

The Endocrine Society 90th Annual Meeting

Therapeutic substitution of levothyroxine alone can be made for a combination of levothyroxine and liothyronine, according to the results from a recent study.

Marina Zemskova, MD, of the National Institutes of Health, in Bethesda, Md., presented the findings Sunday at The Endocrine Society’s 90th Annual Meeting, held in San Francisco.

The researchers conducted a randomized, double blind, crossover study using a three times per day replacement regimen of T3 or T4. The target was thyroid stimulating hormone >0.5 mU/L and <1.5 mU/L following at least 30 days on a stable dose, according to the study. The average age of participants with hypothyroidism (n=7) was 49.6 years; six were women.

No difference was observed for time 0 TSH values between T3 and T4 replacement phases, according to the study. The time 0 TSH was 0.51 mU/L for T3 replacement phase and 0.62 mU/L for T4 (P=.59) at average daily doses of 41.4 mcg for T3 and 123.2 mcg for T4.

Values of total serum T3 concentrations between study phases were larger during T3 than during T4 replacement (P=.002). T3 replacement was an average of 3.0 times higher for coefficients of variation. The TSH responses for the thyrotropin-releasing hormone stimulation test were 281.4 mU/minute/L for area under the curve 0 to 60 (T3) and 282.5 mU/minute/L for AUC 0 to 60 (T4).

“The results suggest there is proof of the concept of effective therapeutic substitution of T3 for T4 therapy,” Zemskova said during the presentation. – by Christen Haigh

For more information:
Zemskova M, Linderman JD, Babar NI, et al. OR11-3. Levothyroxine and liothyronine pharmacodynamic equivalence. A randomized, double blind, cross-over study. Presented at: The Endocrine Society’s 90th Annual Meeting; June 15-18; San Francisco.