In a previous study, FGF21 levels in children and adolescents with HT-caused subclinical hypothyroidism were similar to controls, although the authors assumed that by increasing the number of patients, the tendency of lower FGF21 in subclinical hypothyroidism and the rise of FGF21 after LT4 treatment could reach statistical significance [37]. This evidence concerns the gene FGF21 and hematocrit.