Iatrogenic hyperthyroidism in primary congenital hypothyroidism: prevalence and predictive factors

Primary congenital hypothyroidism (CH) is a preventable cause of mental retardation. Iatrogenic hyperthyroidism has occasionally been reported using the recommended LT4 dosage. Currently, information regarding iatrogenic hyperthyroidism and predictive factors for permanent hypothyroidism (P-CH) among Thai patients is lacking. The aim of this study is to determine the prevalence and factors for predicting iatrogenic hyperthyroidism at one month after LT4 initiation and for predicting P-CH in primary CH infants.

This retrospective cohort study involved 87 infants with primary CH. Patients were classified by thyroid status at one month after LT4 initiation. At 3 years, patients were reevaluated after LT4 cessation and assigned as P-CH or transient CH (T-CH). Differences between groups were analyzed.

One month after LT4 initiation, 35.6% of patients were classified as having iatrogenic hyperthyroidism. An initial LT4 dose of 10.2 μg/kg/day (sensitivity 64.5%, specificity 71.4%) was a suitable cutoff value for predicting iatrogenic hyperthyroidism, wherein 55.6 and 21.6% of patients were treated with initial doses of ≥10.2 and <10.2 μg/kg/day, respectively (p=0.004). Initial LT4 dose was the only predictive factor for thyroid status after initial treatment. At reevaluation, 47.4% of patients were diagnosed with P-CH. LT4 dosage at 3 years of age was significantly higher in patients with P-CH (3.3 vs. 2.85 μg/kg/day, p=0.02) and the only relevant factor for predicting P-CH.

Iatrogenic hyperthyroidism is common among infants with primary CH when treated with the recommended LT4 dosage. LT4 dose was the only factor for predicting iatrogenic hyperthyroidism after LT4 initiation and the diagnosis of P-CH.