Morphological and functional changes in rat thyroid gland after a year following chronic exposure to low and intermediate doses of γ-radiation

INTRODUCTION

Thyroid function depends on iodine uptake by the body as well as on exposure to various harmful environmental hazards (stress, ionizing radiation).

AIM

The aim of the work was to assess the effect of exposure to low and intermediate doses of external γ-radiation on the thyroid structure and function in young female rats at remote periods after radiation.

MATERIALS AND METHODS

Forty female rats were used to study remote effects of external γ-radiation exposure during 20 d (at daily doses of 0.1, 0.25 and 0.5 Gy) on the functional activity (levels of thyroid hormones, iodine metabolism) and the morphological structure of the rat thyroid) after 12 months following the radiation exposure.

RESULTS

An increase in thyroid mass and a decrease in total thyroid protein concentration along with a reduction of blood T3 and T4 was shown only in rat groups exposed to 0.25 and 0.5 Gy. Both the concentration of total iodine and its protein-bound fraction (1.2-1.4 fold, p < .01) and the protein-bound to total iodine ratio were decreased in the thyroids of all irradiated animals. The 0.1-Gy group showed elevated thyroperoxidase (TPO) activity along with increased catalase activity, which may indicate the activation of iodine oxidation by thyrocytes. Only the 0.5-Gy group demonstrated reduced urinary excretion of iodine (2.1 fold, p < .01).The reduction of thyroid function at radiation doses of 0.25 and 0.5 Gy was characterized by a microfollicular structure and the development of atrophic changes in the parenchyma, desquamation of thyroid epithelium and an increase in epithelium proliferation. The diameter of the thyrocyte nuclei was increased in rats exposed to 0.25 and 0.5 Gy, which indicates functional tension of thyrocytes.

CONCLUSION

Our research shows that after a year, the exposure to external γ-radiation of 0.1, 0.25 and 0.5-Gy caused changes in the structure and function of the rat thyroid which are manifested by the development of hypothyroiditis (0.5 Gy), 'subclinical' hypothyroiditis (0.25 Gy) and functional tension of thyrocytes. The mechanisms of thyroid dysfunction - impaired- uptake of iodine and its organification against the background of activation of free radical processes - suggest disturbances in the function of the sodium/iodide symporter (NIS), TPO and thyroglobulin synthesis. In contrast to the intermediate doses, the effects of the 0.1-Gy dose were mostly found at the remote periods compared to the earlier periods (180 days).

[Rat thyrocytes under oxidative stress]

Iodine is the most important regulator of the functional activity of thyroid cells; however, not only may iodine deficiency be responsible for impaired metabolic processes in the thyrocytes. The purpose of the study was to examine the impact of Fe2+/ascorbate-dependent activation ofoxidative stress on the functional activity of cultured rat thyrocytes. The 2.88-6.76-fold rise (p < 0.001) in the concen-tration of aldehyde lipid peroxidation products was shown to be followed by the decreased thyrocytic generation ofthyroxine (Т4) the latter being most pronounced 6 hours late (by 1.7-3.2 times), by a 120-250% increase in triiodothyronine (Т3) excretion and a 1.5-3.0-fold rise in Т3/T4 ratios in the culture medium. This is most likely to result from impaired thyrocytic iodine organisation under activated oxidative stress. The findings suggest that the most important stages of biosynthesis of thyroid hormones in the thyroid are significantly sensitive to the elevated concentrations of active oxygen forms, peroxides, and lipid peroxidation peroxidation products. This factor may make a certain contribution to the pathogenesis of endemic and goiter pathology of the thyroid.