Demonstration of significant epidermal growth factor activity in murine thyroid tissue

Epidermal growth factor (EGF) was first isolated from the mouse submaxillary gland (SMG) which remains its only known site of synthesis. Earlier work suggests that EGF may be implicated in the regulation of thyroid cell growth and proliferation. Moreover, thyroid hormones increase the EGF content of mouse SMG and mimic the maturational effects associated with EGF. The aim of this study was therefore to determine whether the mouse thyroid gland is a site of EGF production. Thyroid and submaxillary glands were homogenized in Tris-HCl, the EGF content was measured by a homologous radioimmunoassay (RIA) and receptor-binding activity assessed in a radioreceptor assay. Epidermal growth factor was readily detectable in each thyroid extract. Dilution curves were parallel to the standard curve. Values obtained for right and left lobes of thyroid were in excellent agreement (r = 0.997, P less than 0.001). The intrathyroidal EGF concentration obtained by RIA was 26.1 +/- 6.0 (S.E.M.) ng/mg protein (n = 40); values obtained by the receptor assay were slightly lower but correlated closely (r = 0.828, P less than 0.01). Assay of homogenates prepared at the same time from the submaxillary glands of these mice showed that thyroidal EGF was not correlated with EGF content in the SMG. These findings indicate that EGF, both biologically active and immunoreactive, is readily detectable in the mouse thyroid and that the thyroid gland is a probable site of EGF synthesis. The methodology provides a model for further studies of the regulation of EGF production and its significance in relation to thyroid disease.