Influence of Long Term Excessive Fluoride Intake on the Morphology and Function of Thyroid in Rats

Objectives To explore the influence of long term excessive fluoride intake on the morphology and function of thyroid in rats.

Methods One hundred and twenty Wistar rats were randomly divided into a control group (tap water, fluoride concentration=0.344 mg/L) and three fluoride exposure groups: low fluoride group (5 mg/L), medium fluoride group (10 mg/L) and high fluoride group (20 mg/L). Ten rats (female: male=1: 1) in each group were sacrificed respectively after 2, 4 and 8 months of exposure. Thyroid glands were weighed, the morphology of thyroid was observed under light microscope. The levels of serum TSH, TT₄, FT₄, TT₃ and FT₃ were detected by radio-immunological assay, and compared among groups.

Results Compared with the control group, serum TSH levels in high fluoride group decreased significantly after 4 and 8 months of exposure (P less than 0.05). Serum TT₄ and FT₄ levels of all fluoride exposure groups increased significantly after 2 months (P less than 0.05), and then began to decrease. No significant changes of TT₃ and FT₃ levels were found after fluoride exposure (P more than 0.05). The follicular hyperplasia and colloid reduction in thyroids were observed by the end of 2 months fluoride exposure, meanwhile, a small number of follicles grew bigger and follicular epithelial cell became flat. With the prolongation of fluoride treatment, follicular hyperplasia became more active, follicular cavity becomes smaller, and fusion giant follicular were observed in some thyroids.

Conclusions Long term excessive fluoride intake could cause abnormal morphology and function of thyroid, and the prolonged time of fluoride exposure seemed to be a key factor.