Protein tyrosine kinases (PTKs) play a role in regulating the growth and differentiated functions of thyroid cells and are probably involved in tumorigenesis of papillary-type thyroid carcinoma. To better understand the roles of PTKs in the physiology and pathophysiology of the thyroid, we analyzed the expression profile of receptor-type PTKs in normal human thyroid tissues. Highly conserved regions in the catalytic domains of receptor-type PTKs were amplified by RT-PCR using degenerate oligonucleotide primers. Nucleotide sequencing of about 100 clones identified 21 PTKs, including 16 receptor type and 5 nonreceptor type; no novel PTK was identified. Insulin-like growth factor I receptor, platelet-derived growth factor receptor (PDGFR), TrkE, Axl, epidermal growth factor receptor, etc., appear to be the most abundant receptor-type PTKs in the thyroid; many of which (PDGFR, TrkE, Axl, etc.) have never previously been demonstrated to be expressed in the thyroid. The expression of messenger RNAs (mRNAs) for PDGFR, axl, and trkE in normal thyroid cells was confirmed by Northern blot analysis, and interestingly, the expression levels of PDGFR and trkE mRNAs were decreased in all three thyroid carcinoma cell lines examined (FRO, WRO, and NPA), whereas axl mRNA and protein were overexpressed in 2 of 3 thyroid carcinoma cell lines (FRO and WRO) compared with that in normal tissue. The axl gene was, however, neither amplified nor rearranged. The biological activity of the ligand for Axl, the product of growth arrest-specific gene 6 (Gas6), was then evaluated, demonstrating modest mitogenic activity in thyroid carcinoma cells overexpressing Axl. Furthermore, gas6 mRNA was expressed in FRO cells. Thus, we here identify a variety of PTKs expressed in the thyroid gland, many of which may participate in the regulation of thyroid cell function. Variable expression levels of some PTKs in normal and cancerous cells suggest that there may be an imbalance and disarray of phosphorylation events in thyroid carcinoma cells. Furthermore, Gas6 is identified as a novel growth factor for thyroid carcinoma cells overexpressing Axl receptor tyrosine kinase.