Increase in doses of levothyroxine at the age of 3 years and above is useful for distinguishing transient and permanent congenital hypothyroidism

There are no recommended diagnostic criteria for transient congenital hypothyroidism (CH) during early childhood. In this study, we aimed to identify the factors that distinguish permanent (P)- and transient (T)-CH. We retrospectively analyzed the clinical, biochemical, and imaging data of 42 children with a definitive diagnosis of P- or T-CH by re-evaluation tests at our institution from November 1986 to October 2019. Patients who continued levothyroxine (L-T(4)) treatment after the re-evaluation tests were classified as group P (n = 19), while patients who were diagnosed with T-CH and discontinued L-T(4) treatment were classified as group T (n = 23). Initial testing performed during infancy showed that the mean serum TSH and free T4 (FT4) levels did not differ significantly between groups P and T. None of the patients in group T required an increased dosage of L-T(4) at the age of 3 yr and above while 85% of the patients in group P required increased dosages of L-T(4). Hence, T-CH was suspected in patients who did not require an increase in L-T(4) dosage at the age of 3 yr and above.