Monozygotic twins are generally discordant for congenital hypothyroidism from thyroid dysgenesis.

In this context, we read with great interest the report by Kuehnen et al. [2] of two pairs of monozygotic twins who are discordant for hypothyroidism due to thyroid dysgenesis. Their report confirms the results of a systematic survey of the Québec and Brussels neonatal screening databases which, together with a review of all published cases since screening began, showed that 12 of the 13 reported pairs (92%) were discordant [3] . Two percent of cases of congenital hypothyroidism from thyroid dysgenesis (CHTD) have an affected relative. This figure is 15-fold higher than what would be predicted by chance alone and clearly suggests a genetic mechanism in those cases [1] . However, it should not be construed as evidence that there is a genetic component in all patients with CHTD, i.e. the 2% with a positive family history and the 98% without may be two discrete populations. Received: June 10, 2009 Accepted: June 24, 2009 Published online: October 19, 2009