[en] Circulating levels of serum human growth hormone (hGH) in 17 normal healthy Thai subjects and 12 patients with Graves' disease of different clinical severity following an oral stimulation of 500 mg L-dopa were determined by radioreceptorassay (RRA) and radioimmunoassay (RIA). The average basal hGH in normal control group was 1.8+-0.8 and 2.4+-1.0 ng/mL (mean+-SD) as measured by RRA and RIA respectively. The mean maximal hGH response to L-dopa occurring between 60-90 min after drug administration was 13.9+-5.0 ng/mL by RRA and 15.6+-6.1 ng/mL by RIA. In Graves' patients, mean basal hGH was 1.4+-0.5 ng/mL by RRA and 1.6+-0.6 ng/mL by RIA with the maximal response of 4.4+-2.4 ng/mL by RRA and 4.6+-2.3 ng/mL by RIA. The hGH response to L-dopa observed in 10 of 12 Graves' patients was significantly lower than that found in 15 of 17 normal control subjects (P<0.005). Although 2 Graves' patients, one in mild and another in moderate state, exhibited good normal response to L-dopa, this study was consistent with other reports that excessive circulating thyroxine level has a direct suppressive effect on hypothalamus-pituitary regulation of hGH secretion. Quantitative comparison of the results obtained from RRA-assayable with those of RIA-assayable hGH showed no discrepancies in the mean ratio of RRA-RIA in all sera of the normal controls as well as patients studied with the correlation coefficient of 0.88. The results of this study indicated that there was a good correlation between biological and immunological activities of hGH circulating in normal subjects and Graves' patients. (author)