Is It Time to Expand Newborn Screening for Congenital Hypothyroidism to Other Rare Thyroid Diseases?

Congenital hypothyroidism (CH) is a heterogeneous condition present at birth, resulting in severe-to-mild thyroid hormone deficiency. This condition is difficult to recognize shortly after birth. Therefore, many countries worldwide have implemented newborn screening (NBS) programs for CH since the 1970s. The most recent European guidelines strongly recommend screening for primary CH, as well as for central CH when financial resources are available. However, no consensus has been reached yet to screen more rare forms of CH, such as Allan-Herndon-Dudley syndrome (AHDS), an X-linked condition linked to mutations in the gene encoding a transmembrane monocarboxylate transporter (MCT8), resistance to thyroid hormone beta (RTH beta), and resistance to thyroid hormone alfa (RTH alpha). The combined measurement of thyroid-stimulating hormone (TSH) and total thyroxine (TT4) on DBS currently allows the recognition of central CH (TSH low/normal and low TT4 without defects in transport proteins). With the introduction of liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) for measurement of free triiodothyronine (FT3) and free thyroxine (FT4), it would be possible to screen for RTH beta (TSH normal/high and high FT4). More complicated would be the method to screen RTH alpha. It would require the combined measurement of FT4 and FT3 and the determination of FT3/FT4 ratio, while the combined measurement of FT3 and reverse T3 (rT3) to calculate FT3/rT3 ratio would be useful to screen AHDS. In this article, we provide some reflections on expanding NBS for primary CH also to other rare forms of CH.