Genetics of Gland- in-situ or Hypoplastic Congenital Hypothyroidism in Macedonia.

Neonatal screening in Macedonia detects congenital hypothyroidism (CH) with an incidence of 1 in 1,585, and more than 50% of cases exhibit a normally located gland- in-situ (GIS). Monogenic mutations causing dyshormonogenesis may underlie GIS CH; additionally, a small proportion of thyroid hypoplasia has a monogenic cause, such as TSHR and PAX8 defects. The genetic architecture of Macedonian CH cases has not previously been studied. We recruited screening-detected, non-syndromic GIS CH or thyroid hypoplasia cases ( n = 40) exhibiting a spectrum of biochemical thyroid dysfunction ranging from severe permanent to mild transient CH and including 11 familial cases. Cases were born at term, with birth weight >3,000 g , and thyroid morphologies included goiter ( n = 11), thyroid hypoplasia ( n = 6), and apparently normal-sized thyroid. A comprehensive, phenotype-driven, Sanger sequencing approach was used to identify genetic mutations underlying CH, by sequentially screening known dyshormonogenesis-associated genes and TSHR in GIS cases and TSHR and PAX8 in cases with thyroid hypoplasia. Potentially pathogenic variants were identified in 14 cases, of which four were definitively causative; we also detected digenic variants in three cases. Seventeen variants (nine novel) were identified in TPO ( n = 4), TG ( n = 3), TSHR ( n = 4), DUOX2 ( n = 4), and PAX8 ( n = 2). No mutations were detected in DUOXA2, NIS, IYD , and SLC26A7 . The relatively low mutation frequency suggests that factors other than recognized monogenic causes (oligogenic variants, environmental factors, or novel genes) may contribute to GIS CH in this region. Future non-hypothesis-driven, next-generation sequencing studies are required to confirm these findings.