Levothyroxine dosages less than 2.4 μg/kg/day at 1 year and 1.3 μg/kg/day at 3 years of age may predict transient congenital hypothyroidism

Congenital hypothyroidism (CH) can be permanent (P-CH) or transient (T-CH), and based on factors such as sex, birth wt, and levels of TSH, T4, free T4 (fT4), and Tg (1,2,3,4,5,6,7,8,9,10,11,12) at the time of neonatal screening tests, these two CH types are difficult to differentiate prior to treatment initiation. However, previous studies have reported significant differences in the levothyroxine (LT4) dosages for 3-yr-old children with P-CH vs. T-CH (1, 3, 5, 7,8,9,10,11,12), and six studies have also identified LT4 dosage cutoff values for differentiating between P-CH and T-CH in 3-yr-old children (7,8,9,10,11,12). The aim of this study was to determine the presently unknown LT4 dosage cutoff values for Japanese children of ages 1 and 3 yr.