Detection of neonates with mild congenital hypothyroidism (primary) or isolated hyperthyrotropinemia: an increasingly common management dilemma.

Anywhere from 10% to 40% of neonates detected by newborn screening programs have mild congenital hypothyroidism (thyroid-stimulating hormone [TSH] 6 to 20 mU/l with borderline low free T4) or isolated hyperthyrotropinemia. The increasing frequency of such cases appears to be chiefly the result of lowering screening TSH cutoffs. In some cases, the etiology is a mild form of dysgenesis or dyshormonogenesis; most cases, however, on imaging have gland in situ of unexplained etiology. Re-evaluation after age 3 years shows some with transient hypothyroidism, a minority with permanent hypothyroidism, while the majority have persistent, mild TSH elevation and normal free T4. There is limited data on neurodevelopmental outcome to guide management. In cases where the TSH is trending down and free T4 is normal, we recommend re-checking serum TSH and free T4 at weekly intervals. If serum TSH does not normalize by 4 weeks of age, we recommend treatment, with re-evaluation after age 2-3 years.