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Levothyroxine Dosage as Predictor of Permanent and Transient Congenital Hypothyroidism: A Multicenter Retrospective Study in Japan

Background: Congenital hypothyroidism (CH) can be divided into 2 types, transient CH (T-CH) and permanent CH (P-CH), depending on the requirement of levothyroxine (LT4) for life-long treatment. Several studies have recently reported that the LT4 dosage is useful for predicting the LT4 requirement, but none of the studies followed their patients to puberty. Objective: To determine the cutoff value for the LT4 dosage as a predictor of the LT4 requirement after puberty in patients with CH. Methods: The LT4 dosage and clinical data on 99 patients with CH who were followed at the participating hospitals from the neonatal period to 15 years of age or older were retrospectively analyzed. Based on their LT4 requirement at their last hospital visit, the participants were divided into the P-CH group (n = 75), who were treated with LT4, and the T-CH group (n = 24), who were not. Results: At age 1 year, a higher LT4 dosage was required for the P-CH group (median 3.75 vs. 2.88 µg/kg/day; p < 0.001). When the LT4 dosage cutoff value at age 1 year was set at 4.79 and 1.74 µg/kg/day, the specificity of P-CH and T-CH (for denying T-CH and P-CH, respectively) was 100 and 97%, respectively. Conclusions: An LT4 dosage above 4.7 µg/kg/day and below 1.8 µg/kg/day at age 1 year may help predict P-CH and T-CH, respectively.