Exposure to nitrite alters thyroid hormone levels and morphology in Takifugu rubripes

Nitrite (NO^2-) can act as a toxic nitrogenous compound with the potential to disrupt endocrine systems in fish. The aim of the present study was to investigate the effects of nitrite on the thyroid endocrine system of Takifugu rubripes. Fish were exposed to 0, 0.5, 1, 3, and 6 mM nitrite concentrations. Blood was collected to assay the concentrations of thyroid-stimulating hormone (TSH), thyroxine (T₄), triiodothyronine (T₃), free thyroxine (FT₄), free triiodothyronine (FT₃), and 3,3,5'-triiodothyronine (rT₃), as well as the activity of iodothyronine deiodinases (Dio1, Dio2, and Dio3,) after 0, 12, 24, 48, and 96 h of exposure to nitrite. The first branchial arch to the third branchial arch of T. rubripes were sampled and fixed, and thyroid morphology was observed. The results showed that exposure to nitrite significantly increased the concentrations of TSH, T₃, FT₃, and reduced the concentrations of T₄, FT₄, and rT₃. The activity of Dio1 and Dio2 increased significantly, whereas Dio3 activity decreased significantly. Additionally, thyroid follicles degenerated and became blurred and most colloid material disappeared 96 h after exposure to high nitrite concentrations. Based on these results, high nitrite concentration exposure can disturb thyroid hormone homeostasis, alter thyroid follicle morphology, and result in thyroid endocrine toxicity.