Time-dependent effects of chlordane on thyroid histological structure and function in rats

In order to study the time-dependent effects of chlordane on thyroid hormone levels and histological structure in rats, 120 healthy 4-week-old SD rats were divided randomly into 12 groups. Rats in treatment groups were treated orally by gavage with chlordane at a dose of 15 mg kg^?1 for 5, 10, 15, 20, 30, or 40 days, while rats in control group received an equal volume of corn oil. Serum levels of thyroxine (FT4), triiodothyronine (FT3), and thyroid stimulating hormone (TSH) were measured by radioimmunoassay. Sections of thyroids were dyed with hematoxylin–eosin to detect any pathological alterations. In chlordane treated groups, on day 5, changes in serum levels of FT4, FT3, and TSH were not found. On days 10 and 15, serum levels of FT4 and FT3 decreased, accompanied by increased levels of TSH. On days 20, 30 and 40, serum levels of these hormones returned to control values. As for alterations in histology, on day 10–15, follicular epithelium of the thyroid were found to proliferate into two layers in chlordane treated rats. On day 20, in focal areas the two-layer afollicular epithelia were observed to continuously proliferate into 3–4 layers of follicular epithelium, termed hyperplasia plaques. On days 30 and 40, solid buds and secondary follicles to solid buds were detected. The earliest alteration of thyroid hormones in treated SD rats occurred between 5 and 10 days with corresponding histopathologic changes. As the exposure time increased, the serum levels of hormones and histopathology altered in a time-dependent manner.