LEI'TERS 1319
autoimmune thyroid disease (4) and, in some of those observations, thyroglobulin was identified as the antigenic comiponent of the nephritogenic immune complexes (4). In our patient, thyroglobulin was not involved in the formation of the cryoglobulin which consisted of monoclonal IgMK complexed with polyclonal IgG. The IgM component exhibited rheumatoid factor activity, which is a usual finding in mixed (type 11) cryoglobulinemia (2). The demonstration of thyroid-stimulating activity concentrated in the IgG fraction would suggest that the same immunoglobulins are involved in thle thyroid disease and in the cryoglobulinemia.
Indeed, thyroid-stimulating immunoglobulins are known to play a major role in Graves' disease, and the formation of mixed (type 11) cryoglobulins is thought to result from specific reactions between monoclonal IgM and selected IgG molecules, with the possible participation of idiotypic interactions (2). The simultaneous occurrence of a monoclonal IgM rheumatoid factor and of cryoprecipitable thyroid-stimulating immunoglobulins in this patient may be fortuitous, but the possible role of B cell hyperactivity, which has been shown to be associated with thyrotoxicosis (3, should be considered.