The application of a cytochemical bioassay to measure thyroid stimulating antibodies; a study of patients with euthyroid Graves' ophthalmopathy.

Using a cytochemical bioassay for thyroid stimulators, we have studied in detail the responses to IgG preparations from sera of 11 patients with euthyroid Graves' ophthalmopathy. This assay is based upon changes in naphthylamidase activity in response to the addition of thyroid stimulators. A wide range of dilutions of each IgG preparation was tested, and bell-shaped dose response curves were obtained where thyroid stimulating antibody (TSAb) was present. Such dose-response curves, giving a maximum response at one dilution (higher or lower concentrations giving a decreased or nil response) are characteristic of this assay. Results were expressed as a titer which is defined as the reciprocal of the dilution giving the maximum response for a given sample. Our routine strategy for titer determination for a given IgG preparation, including the use of "negative" and "positive" controls is described. Retesting of IgG's in separate bioassays showed that the titers were reproducible. Serum from one of the 11 patients was TSAb negative, i.e. did not provoke a response at any dilution tested in this ultrasensitive bioassay. The others were TSAb positive with a wide range of titers (1:10(2) to 1:10(8)). In addition one patient (initial titer 1:10(5)) was plasmapheresed, first in the absence of immunosuppression, when rebound resulted in a titer of 1:5 X 10(7), and secondly in combination with immunosuppression when the titer was reduced to less than 1:10(2). These large changes in titer for samples from one patient were consistent with the strikingly wide ranges of titers found for the 10 other patients.