| Risk Assessment of Thyroid Follicular Cell Tumors Richard N. Hill,1 Thomas M. Crisp,2 Pamela M. Hurley,1 Sheila L. Rosenthal,2 and Dharm V. Singh2 1Office of Prevention, Pesticides and Toxic Substances, and 2Office of Research and Development, U.S. Environmental Protection Agency, Washington, DC 20460 USA Abstract
Thyroid follicular cell tumors arise in rodents from mutations, perturbations of thyroid and pituitary hormone status with increased stimulation of thyroid cell growth by thyroid-stimulating hormone (TSH) , or a combination of the two. The only known human thyroid carcinogen is ionizing radiation. It is not known for certain whether chemicals that affect thyroid cell growth lead to human thyroid cancer. The U.S. Environmental Protection Agency applies the following science policy positions: 1) chemically induced rodent thyroid tumors are presumed to be relevant to humans ; 2) when interspecies information is lacking, the default is to assume comparable carcinogenic sensitivity in rodents and humans ; 3) adverse rodent noncancer thyroid effects due to chemically induced thyroid-pituitary disruption are presumed to be relevant to humans ; 4) linear dose-response considerations are applied to thyroid cancer induced by chemical substances that either do not disrupt thyroid functioning or lack mode of action information ; 5) nonlinear thyroid cancer dose-response considerations are applied to chemicals that reduce thyroid hormone levels, increase TSH and thyroid cell division, and are judged to lack mutagenic activity ; and 6) nonlinear considerations may be applied in thyroid cancer dose-response assessments on a case-by-case basis for chemicals that disrupt thyroid-pituitary functioning and demonstrate some mutagenic activity. Required data for risk assessment purposes is mode of action information on mutagenicity, increases in follicular cell growth (cell size and number) and thyroid gland weight, thyroid-pituitary hormones, site of action, correlations between doses producing thyroid effects and cancer, and reversibility of effects when dosing ceases. Key words: iodide pump, microsomal enzyme induction, 5´-monodeiodinase, risk assessment, thyroid follicular cell tumors, thyroid hormone (T4, T3) , thyroid peroxidase, thyroid-stimulating hormone (TSH) , UDP glucuronosyl transferase. Environ Health Perspect 106:447-457 (1998) . [Online 29 June 1998] http://ehpnet1.niehs.nih.gov/docs/1998/106p447-457hill/ abstract.html Address correspondence to R.N. Hill, Office of Prevention, Pesticides and Toxic Substances, U.S. Environmental Protection Agency (7101) , 401 M Street, S.W., Washington, DC 20460 USA. This paper is dedicated to the memory of Orville E. Paynter, who began this project in 1985. Received 29 September 1997 ; accepted 27 February 1998.
The full version of this article is available for free in HTML format. |
