To assess whether adding liothyronine (LT3) to levothyroxine (LT4) monotherapy normalizes serum thyrotropin (TSH) and thyroxine (T4) concentrations in children with congenital hypothyroidism and central resistance to thyroid hormone.

We retrospectively studied 12 patients with congenital hypothyroidism and central resistance to thyroid hormone (6 treated with LT3+LT4 combined therapy and 6 treated with LT4 monotherapy). In patients receiving combined therapy, we compared serum concentrations of TSH, T4, and triiodothyronine before and after addition of LT3. We used repeated measures analysis to compare thyroid function in participants receiving combined therapy vs monotherapy, while accounting for age and intrasubject correlation.

In patients receiving combined therapy, the addition of LT3 was associated with normalization of mean TSH (9.2 vs 4.5 mIU/L, P = .002), a lower proportion of TSH values greater than 10 mIU/L (35% vs 8%, P = .03), and a decrease in mean serum T4 by 23 ± 9% (P < .001). Compared with patients receiving LT4 monotherapy, patients receiving combined therapy had lower mean TSH (8.5 ± 0.9 vs 4.3 ± 0.4, P < .001), lower odds of TSH elevation greater than 10 mIU/L (OR 0.20, 95% CI 0.10-0.41, P < .001), and lower odds of T4 elevation (OR 0.21, 95% CI 0.04-1.09, P = .06). LT3 treatment did not increase serum T3 levels significantly.

The addition of LT3 to LT4 monotherapy facilitates normalization of both serum TSH and T4 in patients with congenital hypothyroidism and central resistance to thyroid hormone. Larger prospective studies are needed to confirm these findings and to determine the effect of combined therapy on neurodevelopmental outcomes.