Factors influencing the growth of normal and neoplastic thyroid tissue.

TSH activates the adenylate cyclase and the phosphoinositide turnover-protein kinase C-calcium systems in thyroid cells and appears to have an important but variable role in controlling the growth of both normal and neoplastic thyroid tissues. Species differences, experimental conditions, and tissue or tumor cell heterogeneity may account for this variability. Although TSH seems to be an important physiologic growth factor, it is neither the exclusive growth factor for the thyroid gland nor absolutely necessary for the effect of other thyroid growth factors. TSH may work in concert with other growth factors such as EGF. Some growth factors influence thyroid growth through TSH, whereas others do not.