Objective: To determine between timing and LT4 dose which was the more important factor for IQ at 7 years in screened congenital hypothyroidism (CH). Methods: 131 children with CH born from 1979 to 1994 and 30 controls were studied. Mean age at recall: 22.8 ± 1.1 days. Mean initial LT4: 5.6 ± 0.1 µg/kg/day. Results: Optimal global IQ (GIQ; 119.0 ± 1.8) was obtained for a recall ≤15 days. Results for a recall after 3 weeks were lower (107.7 ± 2.4). The IQ of infants treated before 21 days (117.1 ± 1.2) was identical to the IQ of controls (117.0 ± 2.3) whereas the IQ of those treated after this threshold was lower (108.6 ± 1.7). No significant differences for GIQ were observed with various initial LT4 doses. Infants treated with a dose of LT4 ≧6 µg/kg/day had a higher performance IQ (117.3 ± 1.8 vs. 112.8 ± 1.2) compared with those treated with a dose <6 µg/kg/day. The severity of CH and socio-economic levels were similar in all groups. Conclusion: In this study, timing appears to be the more important factor for the intellectual outcome.
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