Epidermal growth factor (EGF) is an important mitogen and its secretion in neonatal animals has been shown to be affected by thyroid hormone levels. EGF in blood of humans is found in both platelets (as reflected in its serum level) and in plasma; its origin in plasma remains unclear. Serum and plasma EGF were studied in a group of patients with thyroid disorders. Twenty hyperthyroid subjects (3M, 17F) aged 37.3 +/- 14.9 years and 10 hypothyroid patients (3M, 7F) aged 58.3 +/- 18.6 years were studied before and after euthyroidism was restored. Before treatment, serum EGF in the hyperthyroid patients was elevated compared to normal controls (501 +/- 376 vs 270 +/- 154 pmol/l, p less than 0.001). After treatment of hyperthyroidism, serum EGF returned to the normal levels (232 +/- 176 pmol/l). In contrast, serum EGF was not significantly different in the hypothyroid subjects either before or after treatment (151 +/- 194 and 237 +/- 153 pmol/l respectively). A significant correlation (r = 0.461, p less than 0.001) between serum EGF and serum-free thyroxine index (FTI) was found when all samples from both untreated and treated hyper- and hypothyroid patients were examined. Multiple regression analysis revealed that both serum FTI and platelet count independently affected the serum EGF levels. Similarly, plasma EGF was also elevated in untreated hyperthyroid patients with a median of 26.4 pmol/l (range less than 16.6-88.0), whereas all normal controls and hypothyroid subjects had unmeasurable levels.(ABSTRACT TRUNCATED AT 250 WORDS)