• A novel mutation was found in thyroglobulin (Tg) gene in Wistar Hannover GALAS rats. • A SNP at the acceptor site of 6th intron caused aberrant expression of Tg mRNA. • Mutant phenotypes, goiter and dwarfism, mimicked those caused by goitrogens. • Bimodal dose-responses in thyroidal weights could be attributed to the Tg mutation. • A genomic DNA-based diagnosing method was developed for detecting the Tg mutation.