Objective: Neonatal screening for congenital hypothyroidism (CH) prevents the serious neuropsychological features of CH, but the question remains whether intelligence and motor skills of CH children treated early are completely normal.
Design: In this report we describe the rare case of two genetically identical twins, only one of whom was affected by CH due to thyroid agenesis. L-Thyroxine (9 μg/kg body weight/day) therapy was initiated at 27 days of age and was adequate throughout the follow-up.
Methods: Neuropsychological evaluation was performed on the twins in parallel from 3 months to 8 years of age.
Results: The CH twin (NB) did not show major neuromotor impairments but, compared with the unaffected twin (EB), she had a slight delay in postural/motor achievements and in language development that completely disappeared at 8 years of age. On standardised tests of intelligence, NB was indistinguishable from control children but, compared with her twin, she had lower IQ scores in most testing occasions up to 7 years of age (NB = 108 vs EB = 115). School achievements of NB did not significantly differ from those of her classmates but, compared with her twin, she scored worse in writing, mechanical reading, verbal memory, and possibly in arithmetic.
Conclusions: Because the twins were genetically and phenotypically identical, were raised in the same environment, and received a similar education, it is concluded that hypothyroidism in utero and in the first neonatal month was responsible for the lower neuropsychological achievements of the CH twin. While foetal hypothyroidism is at present unavoidable, earlier diagnosis and initiation of treatment in neonates with CH are important and highly recommended.