We report a patient with congenital hypothyroidism due to athyreosis complicated by a heterozygous thyroid hormone receptor beta (THRβ) gene mutation (R320L), resulting in a severe resistance to thyroid hormone beta phenotype. The proband inherited the mutant allele from his father, presenting a very mild phenotype. While the precise reason for this discrepancy remains unknown, we postulate the possibility of de novo mutation and mosaicism in the father. Correlating thyrotropin (TSH) with free thyroxine (fT4) allowed us to predict the amount of fT4 required to normalize the proband's TSH, which supported the treatment with high dose of levothyroxine.
Keywords: athyreosis; congenital hypothyroidism; levothyroxine treatment; mutation; resistance to thyroid hormone.