The goitre of the BB/O rat: an animal-model for studying the role of immunoglobulins stimulating growth of thyroid cells.

The BB rat is a well-known animal model for the study of autoimmune thyroid disease. Antithyroglobulin antibodies can be detected in the circulation from the age of 6 weeks onwards, and accumulations of lymphoid cells occurs in the thyroid of up to 60% of animals at the age of 20 weeks and over. The rat, however, stays euthyroid, the thyroid is not destroyed, and hence the disease is not yet well characterized. The study reported here shows that the thyroid weights of 41% (40/97) BB rats were raised from week 6 onwards in comparison to those of Wistar controls. Morphologically, BB thyroids showed a strong similarity to the human disease entity 'colloid goitre', namely an active growth, high columnar epithelium, branching and budding of thyrocytes, no signs of thyroid destruction and in 42% of rats at the age of 22-26 weeks, a development of intrathyroidal lymphoid tissue. Plasma TSH was not significantly raised in the animals. For this reason the presence of immunoglobulins which stimulate the growth of thyroid cells (so-called TGI's) was determined in 12-16-week-old BB rats (n = 10) and in control Wistar rats (n = 10). At this time a significant difference could be recorded in thyroid weights between BB/O rats and Wistar controls (20.1 +/- 6.0 mg vs. 15.8 +/- 2.9 mg, respectively), even in the absence of any intrathyroidal lymphoid cell infiltration. Protein-A-Sepharose purified serum IgG of these animals was used to detect TGI-activity via the 'Feulgen Cytochemical Bioassay'. Of the BB/O rats, eight were clearly positive for TGI, all of the Wistar rats were negative. The data show that the autoimmune prone BB rat may thus serve as an animal model for euthyroid goitre associated with thyroid stimulating antibodies.