ABSTRACT
Kojic acid is a natural substance, widely used as a food additive and preservative, in cosmetic preparations as a skin whitening agent, a plant growth factor as well as a chemical intermediate in pharmaceuticals. The goal of the present study was to clarify the impact of kojic acid on the thyroid follicular cells of male albino rats as well as the reversibility of its effect after withdrawal, both histologically and biochemically. A total of 30 adult male albino rats [120-150 g] were randomly divided among 3 equal groups. Group I [control group]: received isotonic saline orally for 4 weeks. Group II [treated group]: received kojic acid in a dose of 0.8g/kg b.w./day orally for 4 weeks. Group III [withdrawal group]: received kojic acid daily in the same dose and duration as in group II followed by its withdrawal for the next 4 weeks. By the end of the experimental periods, blood samples were collected for estimation of plasma triiodothyronine [T3], thyroxine [T4] and thyroid stimulating hormone [TSH] levels. Histological examination of toluidine blue-stained semithin sections, and ultrathin sections of the thyroid gland was also carried out. Statistical assessment of the biochemical parameters of group II rats showed significant reduction in T3 and T4 levels along with evident increase in TSH levels as compared to the control animals. Histologically, overt signs of hyperactivity affecting many thyroid follicles were encountered. It was represented by follicular cell hypertrophy and focal hyperplasia. Stratjfication of the lining follicular cells was also observed in some follicles. Many follicular cells exhibited dilatation of the rough endoplasmic reticulum [rER], cytoplasmic vacuolation, accumulation of numerous lysosomes and mitochondrial affection. Some follicular cells revealed signs of severe degeneration in the form of dense cytoplasm and small irregular dense nuclei. Withdrawal of kojic acid in group III rats resulted in return of the biochemical parameters as well as histological picture of the follicular cells to nearly normal control patterns. However, some follicular cells still revealed evident dilatation of the rER and many lysosomes. As well, few follicles were lined by more than one layer of follicular cells. Kojic acid exerted an evident toxic effect on the thyroid follicular cells. Although such effect was nearly completely reversible after kojic acid withdrawal, yet, it is recommended to restrict its use in food and cosmetic agents