Responses to TRH and T3 suppression tests in euthyroid subjects with a family history of Graves' disease.

The relationship of Graves' disease and heredity was studied in 97 clinically and biochemically euthyroid relatives (resin T3 uptake and serum T3, T4, and TSH within normal ranges) who had more than two thyrotoxic relatives within the second degree relationship. TRH tests were preformed in all 97 cases. In 56 of the 97, T3 suppression tests were performed shortly after the TRH test. Results revealed that 29 of the 97 (29.9%) showed an abnormal response to TRH. fourteen of these (14.4%) revealed no response or a hyporesponse, and 15 (15.5%) revealed a hyperresponse to TRH. Four of 56 (7.1%) were T3 nonsuppressible. Seven individuals who showed no response or a hyporesponse to TRH consisted of 2 nonsuppressible and 5 suppressible subjects. In 14 non- or hyporesponsive cases, serum T3 (1.51 +/- 0.05 ng/ml; mean +/- SE) and T4 (9.91 +/- 0.31 micrograms/dl) were significantly higher compared with those of normal responders (1.30 +/- 0.04 ng/ml, 8.57 +/- 0.21 micrograms/dl; P less than 0.001) or hyperresponders (1.16 +/- 0.06 ng/ml, 7.77 +/- 0.63 micrograms/dl; P less than 0.01). There was no correlation between TRH responsiveness and T3 suppressibility. A relatively high occurrence of thyroglobulin and microsomal antibodies was observed, further suggesting a hereditary predisposition. The findings indicate that even in euthyroid relatives with a family history of Graves' disease who have no clinical or biochemical abnormalities of thyroid dysfunction, many have abnormalities in TRH responsiveness, T3 suppressibility, and thyroidal antibodies.