Regressive and non-regressive thyroid lesions of the rat induced by single injection of N-bis(2-hydroxypropyl)nitrosamine and iodine deficient diet.

Six-week-old male F344 rats were divided into 4 groups. Rats in Groups 1 (n = 16) and 3 (n = 14) received a s.c. injection of N-bis(2-hydroxypropyl)nitrosamine (DHPN) (2800 mg/kg) in experimental week 1 while rats in Groups 2 (n = 5) and 4 (n = 5) received saline. From weeks 2-20, all rats were given an iodine deficient (I-def) diet and tap water. Groups 1 and 2 were killed for the measurements of thyroid-stimulating hormone (TSH), thyroxine (T4), the maximum thyroid width (MTW), thyroid weight, morphology, morphometrics and proliferating cell nuclear antigen (PCNA) labeling index (LI). The thyroids of the rats in Group 3 and 4 were surgically exposed and the MTWs were measured. These latter rats were given basal diet for 6 weeks to recover from iodine deficiency, and then killed for the same measurements. Thyroid nodular lesions in Group 1 rats were classified into five categories (NL0, NL1, NL2, NL3 and NL4) based upon incremental cellular and structural atypia. Two types of regressive nodules (NL'0 and NL'1+2) were identified in the recovered rats as the regressed form of NL0, NL1 and NL2 lesions. NL3 and NL4 nodules were seen in Groups 1 and 3. The mean number of combined NL0, NL1 or NL2 lesions was 28.44 +/- 6.12 nodules per rat (NPR) in Group 1 rats and the mean number of NL'0 and NL'1+2 lesions was 28.07 +/- 13.05 NPR in Group 3 rats. The mean number of NL3 or NL4 lesions was 1.70 NPR in Group 1 rats and 3.42 NPR in Group 3 rats. The LIs were NL0 (6.4 +/- 2.5%), NL1 (7.7 +/- 4.4%), NL2 (0.7 +/- 0.3%), NL3 (7.5 +/- 1.3%) and NL4 (14.4 +/- 5.3%) in Group 1 rats and NL'0 (< 0.001%), NL'1 + 2 (< 0.01%), NL3 (9.0 +/- 4.4%) and NL4 (23.3 +/- 17.8%) in Group 3 rats. The thyroid weights of Group 4 rats were 41% of Group 2 rats. The volume fraction (VF) of the non-NL3, non-NL4 areas in Group 3 rats was 40% of that in Group 1 rats. However, the VF of NL3 or NL4 lesions in Group 3 rats was 520% of that of Group 1 rats. In summary, the growth of the NL0, NL1 and NL2 lesions was TSH-dependent, whereas NL3 and NL4 lesions were TSH-independent.(ABSTRACT TRUNCATED AT 400 WORDS)