Solid versus Liquid Formulation of Levothyroxine: Comparison of Effectiveness in Newborns with Congenital Hypothyroidism

Introduction: Recent studies have found an incomplete bioequivalence between solid and liquid formulations of levothyroxine sodium both in pediatric patients and adults with reduced gastric pH.^1–3 The objective of our study was to retrospectively compare the effectiveness of the solid and liquid formulations of levothyroxine in newborns with congenital hypothyroidism during the first month of therapy.

Materials and Methods: We retrospectively collected data from 46 newborns with congenital hypothyroidism followed up at our tertiary care hospital. Group A (n = 23) received solid formulation of levothyroxine sodium and group B (n = 23) was treated with a liquid formulation (10.0 vs. 10.8 µg/kg/die, p = 0.11). The two groups did not significantly differ for gestational age, birth weight, and had no differences in age and thyroid-stimulating hormone (TSH) values at the first and second screening assay. Eventually, at the day of the definitive diagnosis, there was no difference in age, TSH values, FT4, FT3, antithyroid antibody titer, and etiology of hypothyroidism, nor were different weight and levothyroxine dosage at the beginning of treatment. Patients were observed for the first 15 days and the first month of therapy, and data were collected about weight, levothyroxine dose, and serum TSH and FT4 values from each follow-up visit performed.

Results: The mean values of fT4 after the first month of treatment were significantly lower in the patients treated with liquid formulation than in those treated with a solid one, respectively, 24.0 versus 17.9 ng/L (p = 0.009). All other studied variables, namely, TSH (38.3 vs. 1.3; p = 0.16) and fT4 values at treatment day 15 and TSH at 1 month (11.2 vs. 1.1; p = 0.17) did not show significant differences.

Discussion: Our data, with the limitation of the retrospective nature of the study, show that in newborns with congenital hypothyroidism the liquid formulation allows a better achievement of the therapeutic goals proposed by the current AAP and ESPE guidelines^4,5 in the first month of the treatment. The liquid formulation should therefore be preferred when prescribing such treatment early in life.