Effect of altered thyroid status on the incidence of colon tumors induced by methylnitrosourea in rats

Aim. The present study was designed to determine whether medically induced hyper- and hypothyroidism effect on incidence of colon tumors induced by methylnitrosourea (MNU) burden in rats. Methods. Female rats ( n = 88) were randomly divided into four groups: I (euthyroid-control), II (hyperthyroid caused by liothyronine), III (hyperthyroid caused by L-thyroxine) and IV (hypothyroid caused by propylthiouracil (PTU), also 11 rats were intact control. Colon carcinogenesis was induced with a four intrarectal instillation of MNU (4 mg in 0.5 ml saline solution) one time per week. Liothyronine (100 ± 10 µg per 100 g of animal weight 1 time per day), L-thyroxin (100 ± 10 µg per 100 g of animal weight 1 time per day) and propylthiouracil (PTU, 2,0 ± 0,15 mg per 100 g of animal weight 1 time per day) were administered intragastrically through an atraumatic probe daily, starting from the day of the last intrarectal instillation of MNU. Rats were sacrificed at 216 days after experiment beginning, and the total colon were excised, fixed for histology and analyzed. Results. Drug inhibition of thyroid hormone function by PTU resulted in a decrease in the incidence of MNU-induced colon tumors and amounted to 27.3%. The incidence of colon tumors in the hyperthyroid group caused by L-thyroxine was 70.0% ( F -test – 0.012, χ 2 – 7.67; p < 0.05 compared with the hypothyroid group).