NGF and thyroid gland carcinoma: Interestingly, the tyrosine kinase receptor TrkA, whose ligands include both NGF and proNGF, is expressed in nerves that have infiltrated the tumour microenvironment of thyroid cancer20, suggesting the hypothesis that proNGF/NGF could stimulate nerve outgrowth via signalling through TrkA receptors in nerve terminals infiltrated in thyroid cancer.