Pseudohypoparathyroidism type la (PHP-1a) is an uncommon disorder that results from an inactivating mutation in the GNAS gene. It can present with resistance to several hormones, in addition to parathyroid hormone (PTH). Patients may have the classic Albright's hereditary osteodystrophy (AHO) phenotype and can develop resistance to thyroid stimulating hormone (TSH), gonadotropins, growth hormone releasing hormone (GHRH), and other hormones that rely on the Gsalpha protein to regulate signal transmission at their receptors. We report two siblings with PHP-1a and congenital hypothyroidism. The patients were found to have a heterozygous mutation at nucleotide 305 in exon 4 (c305C-->A) of the GNAS gene, which has not been previously linked to congenital hypothyroidism.