Endocrine Abstracts

Introduction: Neonatal screening for congenital hypothyroidism is considered a major cornerstone in making early diagnosis and starting therapy in neonates. This is a very important step to prevent the consequences of untreated hypothyroidism including mental retardation and developmental delay. Neonatal screening is usually conducted by screening the level of T4 with or without TSH level. Confirming the diagnosis of hypothyroidism is a very important step before starting therapy.

The case: A 6-week-old male presented for evaluation of congenital hypothyroidism. He was diagnosed by neonatal State screen for a reported low T4 and was started on Levothyroxine therapy of 10 micrograms po daily at the age of two weeks. He has been irritable and had diarrhea in the past ten days which persisted despite changing the milk formula. His physical examination revealed tachycardia and brisk reflexes. Laboratory obtained during the visit findings showed: TSH 0.003 uIU/ml (0.6 -3.75 uU/ml): 0.003 mIU/l T4 5.1 ug/dl (4.5-12 ug/dl): 65.64 nmol/l Free T4 3.2 ng/dl (0.8- 1.9 ng/dl): 41.2 pmol/l Based on clinical presentation and laboratory results, thyroxine-binding globulin (TBG) deficiency was suspected and confirmed by an additional laboratory test, the Levothyroxine therapy was discontinued, the parents were educated about the diagnosis, and repeated thyroid function tests obtained one month later revealed normal TSH and Free T4.

Conclusion: Positive neonatal screening tests need to be confirmed with repeated laboratory tests before starting therapy. Making the correct diagnosis of neonatal congenital hypothyroidism is a very important step too. Understanding the differential diagnosis of normal TSH Low T4 is essential. When the Free T4 level is normal, considering and confirming TBG deficiency diagnosis is essential since over-treatment with Levothyroxine can cause craniosynostosis. If the free T4 level is low, considering secondary congenital hypothyroidism is needed while obtaining full pituitary work -up including the evaluation of the ACTH-Cortisol axis. Early treatment with Levothyroxine in this case may unmask adrenal insufficiency. The neonatal thyroid screen interpretation requires a detailed understanding of the thyroid function tests normal values in the first weeks of life.