We studied thyroid tissue from two siblings with Pendred's syndrome (familial goiter and congenital deafness), both with the Mondini-type inner ear malformation, goiter, and hypothyroidism. Iodine trapping and peroxidase levels were grossly normal. Thyroglobulin (Tg), the only iodoprotein found, had a normal monomer size (330 kilodaltons), but low content of hormone and iodine. Tg's expected N-terminal peptides of 26 and 18 kilodaltons, usually formed in association with iodination and thyroid hormone synthesis, were absent, but appeared after iodination in vitro. Reverse transcription of ribonucleic acid from Pendred thyroid tissue and amplification by polymerase chain reaction of specific regions encoding the most important hormonogenic sites of Tg revealed a normal complementary DNA sequence corresponding to the first 100 amino acid residues in Tg's N-terminus. However, 3 of 35 clones of the 3'-region corresponding to the Tg C-terminus exhibited a deletion of nucleotides 7860-7994; this deletion was not present in any of the 150 clones from 7 other thyroids we examined. Four Pendred clones had a 2-nucleotide deletion at positions 7870-7871, a change that would result in a premature stop codon and was found in thyroids from several other subjects as well. We conclude that the messenger ribonucleic acid encoding the 3'-region of Tg can be abnormal in Pendred's syndrome. Some, but not all, of these changes also occur in other human thyroids. Further work is necessary to show if and how these alterations relate to defective hormone synthesis and goiter.