Treatment of hypothyroidism with the combination of levothyroxine and slow-release triiodothyronine: a randomized clinical trial

Background

Some patients with hypothyroidism lack satisfaction with levothyroxine (LT4) monotherapy, which may be related to lower serum triiodothyronine (T3) and T3/T4 ratios compared to control individuals. This study aimed to evaluate the efficacy and safety of a combination therapy of slow-release T3 (SRT3) and LT4 in patients with primary hypothyroidism compared with LT4 monotherapy.

Methods

Thirty-two hypothyroid women were randomized into two groups of SRT3 + LT4 combination and LT4 monotherapy. Group one received a combination of 15 µg SRT3 and 75 µg LT4, and group two received 100 µg LT4 daily for 8 weeks. Clinical and biochemical measurements were performed at baseline and 4 to 8 weeks after intervention.

Results

There were no significant changes in serum levels of fT4, T3, TSH, and T3/fT4 ratio in the LT4 group at the end of 4 to 8 weeks of study. A statistically significant decrease in fT4 and TSH, and an increase in serum T3 and the T3/fT4 ratio, were observed in the SRT3 + LT4 group. The T3/fT4 ratio reached comparable values to those in normal subjects, 93.63 ± 23.25 vs 95.06 ± 19.44 ng/ng, respectively. The rise in the T3/fT4 ratio 8 weeks after SRT3 + LT4 treatment was between 21 % and 90 % in 10 patients and 1 % and 13 % in 5 patients, with no change in one patient.

Conclusion

The novel combination of SRT3 + LT4 therapy resulted in a significant increase in serum T3 and the T3/fT4 ratio in hypothyroid patients compared to those receiving LT4 monotherapy. The rise in the T3/fT4 ratio was ≥ 21 % in two-thirds of patients; the lack of a significant increase in the T3/fT4 ratio in some patients during SRT3-LT4 combination therapy demands further investigation.