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Thyrotropin (TSH) is the main growth factor for the thyroid gland although
several others exist, some of which such as epidermal growth factor and
insulinlike growth factor-I are probably required for TSH to induce its
growth stimulating effect(2). Animal models have demonstrated also a carcinogenic
effect of endogenous TSH when the thyroid hormone synthesis from the gland
was blocked by antithyroid drugs.
A multistep model for thyroid tumorigenesis has been suggested by Wynford-Thomas et al(4) and Farid et al.(53 involving activation of oncogenes, inactivation of tumour suppressor genes as well as other genetic events. These models imply possible development from a benign to a malignant thyroid tumour e.g. from a follicular adenoma to a follicular carcinoma by several somatic mutations. In the familial form of medullary carcinoma (multiple endocrine neoplasia [MEN 2A]) germ-line mutations have recently been found(6), and this has markedly improved the identification of MEN 2A gene carriers and thereby early treatment by thyroidectomy(7). A relationship between radiation and development of thyroid neoplasms has been demonstrated in several contexts. One of the clearest proofs that radiation causes thyroid cancer is the demonstration of a dose-response relationship between the two(l). The thyroid is more sensitive to radiation in childhood and population based studies have thus confirmed a relationship between radiation of the neck in childhood and increased risk of thyroid carcinoma in adulthood(2). The thyroid is the organ most sensitive to the neoplastic effect of radiation, most clearly demonstrated by the increasing number of childhood thyroid carcinomas appearing in the areas around Chernobyl after the nuclear fall-out in 1986(8), and even the delayed development of latent thyroid carcinomas in adults surviving exposure to atomic bomb radiation in Hiroshima in 1945(9). The childhood carcinomas from the areas around Chernobyl are mainly papillary, and of an aggressive type as has been seen more often in juvenile papillary carcinomas(8) Further research is necessary into the dose response relationship between delivered irradiation and thyroid carcinoma development in the Chernobyl accident. |
