| Although
feline hyperthyroidism has become a commonly diagnosed disorder of older
cats, the underlying etiology remains unknown. Pathological findings of
adenomatous hyperplasia involving both thyroid lobes in most hyperthyroid
cats suggests the possibility that feline hyperthyroidism may be similar
to human Graves' disease, which results from high circulating levels of
thyroid stimulating immunoglobulins (TSIs). To exclude high circulating
levels of TSIs as the cause of feline hyperthyroidism, we measured intracellular
concentrations of cyclic adenosine monophosphate (CAMP) in functioning
rat thyroid cells (FRTL-5) incubated with IgG extracted from hyperthyroid
cat serum. Since TSIs stimulate thyroid hormone secretion through activation
of cAMP, their presence can be evidenced in vitro by generation of high
CAMP concentrations in cultured thyroid cells. No significant difference
was found in intracellular cAMP concentrations in FRTL-5 cells incubated
with IgG from normal versus hyperthyroid cats. In contrast, IgG from a
human patient with Graves' disease caused substantially more cAMP generation
than either normal human IgG or IgG from the cats of this study. These
results indicate that feline hyperthyroidism does not result from high
circulating concentrations of TSI and, in that respect, is not analogous
to Graves' disease. |