Primary congenital hypothyroidism is characterized by low levels of circulating 
thyroid hormones and raised levels of thyrotropin at birth. It can be either 
permanent or transitory. Most permanent cases (80-85%) result from alterations 
in the formation of the thyroid gland during embryogenesis (thyroid dysgenesis), 
and several were shown recently to be produced by mutations in genes responsible 
for the development of thyroid follicular cells (TITF1, TITF2, PAX8 and TSHR). 
Less frequently, congenital hypothyroidism is determined by defects in thyroid 
hormone synthesis (hormonogenesis defects). The latter are usually associated 
with goiter. Recently, the molecular mechanisms of two forms of hormonogenesis 
defects (iodine transport defects and Pendred syndrome) were elucidated.