Thyroglobulin (Tg) provides the peptide backbone for synthesis for thyroid hormones. Because previous studies by various techniques have raised the possibility of heterogeneity in Tg's message and translated protein, we have applied a highly sensitive ribonuclease protection assay (RPA) to examine the mRNA species translating part of Tg's C-terminal region, an area containing three of Tg's hormonogenic sites. Tissue samples were obtained from 18 normal and diseased human thyroids at surgery. Three probes spanning part or all of the nucleotide segment containing bases 7808-8086 in the cDNA sequence, detected full-length mRNAs as the dominant transcripts but also showed the consistent presence of at least seven discrete smaller mRNA species in the thyroid samples. The amounts of these smaller mRNAs varied among tissue samples without a discernible relationship to the underlying clinical thyroid condition. We conclude that the mRNA for this region of Tg is quite heterogeneous and offers potential opportunities for translation of different peptide sequences that might affect hormonogenesis in the C-terminal region of the protein.