169 THE ETIOLOGY OF CONGENITAL HYPOTHYROIDISM (CHT) IN THE NETHERLANDS

The Dutch national thyroid screening (measuring T4 and in the 20% samples with lowest T4 also TSH) was started in 1981. The abnormal screening results were evaluated 4 till 6 years later. Of the 176,311 screened children (98.7% of all neonates in 1981) 1637 (0.93%) had been referred. Especially the cases with elevated TSH levels (> 25 mU/l in blood spots and/or >10 mU/l in venous samples) were re-examined; in case of a permanent CHT of unknown cause the T4 replacement therapy was interrupted for investigation. Permanent primary CHT was found in 51 cases (incidence 1:3500): 14 ageneses, 3 cryptic thyroids, 27 sublingual thyroids, 4 total organification defects, 1 Pendred syndrome and 2 other dyshormonogeneses. Secondary/tertiary CHT was found in 6 cases (1:30,000): the 3 having an abnormal screening result were already under pediatric control since birth; in the other 3, having normal screening values, the diagnosis congenital TSH deficiency was made after referral for growth retardation. Transient primary CHT, comprising 89 cases (1:2000), outnumbers permanent CHT. In 45 cases a plausible etiology was found retrospectively: one mother used PTU during pregnancy, one child with a congenital nephrotic syndrome and 43 cases in which the fetus or neonate was exposed to high doses of iodine. Severity and duration of the CHT in the other 44 cases were comparable, however the cause remained unknown. In conclusion, the Dutch overall incidence of CHT amounts to 1:1200.