Congenital hypothyroidism

What it is

Congenital hypothyroidism, also called cretinism, is one of the disorders every newborn is screened for: every newborn must be screened for hypothyroidism by ordering TSH. It is also one of the important conditions to be screened for at birth in a particular group of newborns rather than in every newborn: for a neonate under work up for dysmorphic features such as hypertelorism, upslanting eyes and epicanthic folds, whose diagnosis is confirmed by chromosomal culture or karyotyping, the conditions to be screened for at birth are congenital heart disease, congenital hypothyroidism, congenital leukaemia and sensorineural hearing defects.

Detection of congenital hypothyroidism is important because it is two things at once: relatively common, occurring in 1 in 4000 births; and one of the few preventable causes of severe learning difficulties.

Causes and risk

Maldescent of the thyroid and athyreosis is the commonest cause of sporadic congenital hypothyroidism. In maldescent the thyroid remains as a lingual mass or a unilobular small gland. Dyshormonogenesis, an inborn error of thyroid hormone synthesis, accounts for about 5-10% of cases. Iodine deficiency is the commonest cause of congenital hypothyroidism worldwide, and it can be prevented by iodination of salt in the diet. Hypothyroidism due to TSH deficiency is the fourth listed cause. Isolated TSH deficiency is rare, under 1% of cases, and is usually associated with panhypopituitarism, manifesting with growth hormone, gonadotrophin and ACTH deficiency leading to hypoglycemia or micropenis and undescended testes. Hypothyroidism is common in Down syndrome, in 1% of newborns, 10% of children and up to 50% of adults, and there it may be congenital or acquired hypothyroidism from Hashimoto thyroiditis.