Thyroid growth in vitro

Whereas there is no doubt about the role of the growth factors EGF (epidermal growth factor) and IGF I (insulin like growth factor I) on thyroid cell proliferation, controverse results have been obtained in different in vitro studies with TSH (thyrotropin). Whereas in nearly all cell culture systems using thyroid cell monolayers TSH enhances cell proliferation, TSH inhibits cell growth induced by EGF in intact thyroid follicles. These differences are not due to species specificity, because i.e. growth of porcine thyroid cells or of human thyroid cells in monolayer cultures is stimulated by TSH, but growth of human or porcine thyroid follicles is not stimulated by TSH. In addition there is some evidence that IGF I is necessary for the growth stimulating effect of TSH and IGF I seems to be an autocrine growth factor, which is under the control of TSH and/or iodine deficiency within the thyroid cell.