Growth Regulation of Normal Thyroids and Thyroid Tumors in Man

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Abstract
The biological importance of endocrine and paracrine growth stimulation of human thyroid gland and human thyroid tumors is still debated controversially. Most clinical data indicate a mitotic effect of thyrotropin (TSH) on thyrocyte growth in man (Abe et al. 1981; Clark et al. 1983; Goretzki and Clark 1988), whereas investigators using porcine follicles in culture question the direct effect of TSH on thyrocyte growth (Gaertner et al. 1985; Wanabe et al. 1985; Westermark et al. 1983, 1986). In these cells local active growth factors such as epidermal growth factor (EGF) stimulate cell proliferation (Westermark et al. 1986). In FRTL-5 cells, a permanent follicular thyroid cell clone from a rat thyroid tumor (Filetti et al. 1988; Jin et al. 1986; Tramontano et al. 1988), murine (Mothersill et al. 1984; Tamura et al. 1981) and canine thyrocytes (Roger et al. 1984; Roger and Dumont 1982) TSH proved to increase cell growth, which is at least partially related to TSH stimulation of adenylate cyclase (AC; Roger and Dumont 1982). Most of these studies, however, are of limited conclusiveness for humans. Thyrocytes demonstrate species-specific differences in growth regulation, and this requires the study of human thyrocytes. Additionally, using primary cell cultures these cells lack absolute purity because of fibroblast contamination, which can be excluded only by establishing defined permanent cell lines.