Comparison between Liquid and Tablet Formulations in the Treatment of Congenital Hypothyroidism up to 3 Years of Age: The First Italian Study

<b>Background/Objectives</b>: Levothyroxine (L-T4) is available for use in congenital hypothyroidism (CH) in three formulations: tablets, drops, and oral solution. This study aims to compare the efficacy and safety of all three L-T4 formulations. <b>Methods</b>: We enrolled 63 children born between January 2019 and April 2023 in the Emilia-Romagna Region (Italy) and diagnosed with CH by newborn screening. They were divided according to the L-T4 formulation used: drops (Group D), oral solution (Group S), and tablets (Group T). Clinical and laboratory data were collected up to 3 years after the start of replacement therapy. <b>Results</b>: Serum-free thyroxine (sFT4) and thyroid stimulating hormone (sTSH) normalization occurred within the first month of treatment in most patients of all groups. No negative effects on growth and cognitive development were observed. At 7-15 days we found higher median sTSH levels (<i>p</i> = 0.031) and a greater percentage of patients with sTSH > 5 µU/mL (<i>p</i> = 0.011) in Group S than in Group T, but comparable sFT4 levels. At 12 months, a greater percentage of patients of Group D showed sFT4 values below the normal range than Group S (<i>p</i> = 0.011) and Group T (<i>p</i> = 0.038); <b>Conclusions</b>: Overall, our study reported an equal efficacy of the L-T4 oral solution compared to drops and tablets in CH treatment. A larger series of patients and a long-term follow-up are needed.