The Tgncog mutation causes the development of goiters due to failed processing of thyroglobulin. Homozygotes are smaller in overall size by 15 days of age. They have an increase in growth rate at the time of weaning but generally do not attain comparable size with their wildtype littermates. Increased thyroidal volume is apparent at embryonic day 18 and continues enlarging to an average of 5 fold higher than normal at 8 weeks of age and 20 fold normal at 10 months of age. In addition to decreased serum T3 and T4 levels, homozygotes have increased serum thyroid stimulating hormone levels, reduced levels of serum IGFBP-3, IGFBP-4, and IGFBP-2, mild anemia, and hypomyelination restricted to the cerebrum. Tgncog is an outwardly recessive mutation but microdissection reveals a heterozygous phenotype as well. Thyrofollicular cells of heterozygotes have swollen protein-containing vesicles similar to but more moderate than those found in homozygotes. Thyroid extracts from homozygotes have an increased percent of protein and heterozygotes have an intermediate percent of protein relative to wild type siblings. (Beamer et al., 1987; Adkison et al., 1990; Sugisaki et al., 1991, 1992, and 1993; Kim et al., 1996 and1998.)
