Effects of Microsomal Enzyme Inducers on Thyroid-Follicular Cell Proliferation, Hyperplasia, and Hypertrophy

• Published in: Toxicology and applied pharmacology 1999-10, Vol.160 (2), p.163-170
• Main Authors: Hood, Alan, Hashmi, Raj, Klaassen, Curtis D.
• Format: Article
• Language: English
• Subjects: Animal tumors. Experimental tumors; Animals; Biological and medical sciences; Cell Count; Cell Division - drug effects; Chlorodiphenyl (54% Chlorine) - pharmacology; Enzyme Activation - drug effects; Experimental endocrine tumors; Hyperplasia; Hypertrophy; Male; Medical sciences; Methylcholanthrene - pharmacology; Organ Size; Phenobarbital - pharmacology; Pregnenolone Carbonitrile - pharmacology; Rats; Rats, Sprague-Dawley; Thyroid Gland - drug effects; Thyroid Gland - metabolism; Thyroid Gland - pathology; Thyrotropin - blood; Thyroxine - blood; Triiodothyronine - blood; Tumors
• ISSN: 0041-008X; 1096-0333
• DOI: 10.1006/taap.1999.8752

The microsomal enzyme inducer (MEI), phenobarbital (PB), has been proposed to promote thyroid tumors by increasing the biotransformation and elimination of T4, resulting in an increase in serum thyroid-stimulating hormone (TSH). In turn, TSH stimulates thyroid gland function, growth, and ultimately neoplasia. The dose-dependent effects of MEI on thyroid-follicular cell proliferation, a measure of thyroid gland growth, has not been reported. In the present study, it was hypothesized that MEIs that increase TSH would stimulate thyroid-follicular cell proliferation and the total number of thyroid-follicular cells. Male Sprague–Dawley rats were fed either a basal diet or a diet containing PB (at 300, 600, 1200, or 2400 ppm), pregnenolone-16α-carbonitrile (PCN) (at 200, 400, 800, or 1600 ppm), 3-methylcholanthrene (3MC) (at 50, 100, 200, or 400 ppm), or Aroclor 1254 (PCB) (at 25, 50, 100, or 200 ppm) for 7 days. PB and PCN increased TSH 65% and 95%, respectively, whereas 3MC and PCB did not appreciably affect TSH. PB and PCN increased thyroid-follicular cell proliferation 625% and 1200%, respectively, whereas 3MC and PCB did not have a consistent or appreciable effect. The total number of thyroid-follicular cells was not significantly increased by MEI treatment. In conclusion, small increases in TSH by PB and PCN produced large increases in thyroid-follicular cell proliferation, which did not result in a comparable increase in the total number of thyroid-follicular cells. Furthermore, MEI that did not increase TSH did not consistently or appreciably increase thyroid-follicular cell proliferation or cell number.