Levothyroxine replacement in primary congenital hypothyroidism: the higher the initial dose the higher the rate of overtreatment

Background: Congenital hypothyroidism (CH) is the most frequent endocrine disorder during the neonatal period, and a delay in diagnosis and treatment leads to irreversible complications. A high L-thyroxine (LT4) dose is recommended for treatment, while the optimal starting dose is still a matter of debate. The objective of this study was to determine the effects of various starting doses of LT4 on serum thyroid stimulating hormone (TSH) and thyroxine (fT4) at the end of the first month of treatment. Methods: A total of 71 patients (37 males, 52.1%) with CH were included in the study. The patients were designated into three sets of subgroups according to the following categorical variables: (i) initial LT4 doses: 6-9.9 mu g/kg/day, 10-11.9 mu g/kg/day and 12-17 mu g/kg/day; (ii) initial TSH levels: 6-9.99 mu IU/mL, 10-75 mu IU/mL and >75 mu IU/mL; and (iii) etiology: dyshormonogenesis and dysgenesis. A fT4 level >2.3 ng/dL +/- a TSH level <0.5 mu IU/mL were considered as overtreatment. Results: The mean age of the study population was 22.3 +/- 13.2 days at diagnosis. At diagnosis, the mean fT4 was 0.84 +/- 0.32 ng/dL, and TSH was 39.3 +/- 30 mu IU/mL. The mean initial dose of LT4 was 10.9 +/- 2.9 mu g/kg/day. Overtreatment rates were significantly higher in the highest dose group (12-17 mu g/kg/day) compared with the lowest dose group (6-9.9 mu g/kg/day) (61.5% and 25%, respectively, p < 0.05). None of the patients was undertreated. Conclusions: In this study, we found that the rate of overtreatment was significantly higher in patients who were given LT4 doses of 12-17 mu g/kg/day. Thus, monitoring thyroid functions earlier than one month of treatment is necessary.