Abstract
Artificial light provides many benefits to humankind, allowing fundamental activities to continue during night; however, it also poses multiple risks to humans and wildlife and is recognized as a significant driver of global environmental change. Changes in natural light/darkness cycles caused by artificial light at night (ALAN) can affect amphibians, the most threatened vertebrate group globally. The aim of this study was to determine the effects of long-term exposure to constant nighttime light on the morphology of the thyroid glands of Triturus ivanbureschi metamorphosed juveniles using histological analysis. A cool LED light with a color temperature of 6000 K was selected, as this spectrum is commonly used in outdoor lighting. Larvae were raised at a natural day-night light regime. After metamorphosis, juveniles were randomly divided into a control group maintained under natural dark nighttime conditions (<0.1 lux) and a treatment group exposed to LED light (30 lux) at night for 60 days. Standard histological techniques (H&E) and immunohistochemical (IHC) staining were used to examine the thyroid glands. There were no significant differences in the absolute volume densities between the light-treatment and control groups; however, subtle morphological variations were observed. Immunohistochemical analysis of Tg immunostaining revealed a significant difference between the light-treatment and control groups, indicating that the thyroid gland of newts exposed to light has a stronger signal, suggesting the accumulation of thyroglobulin at the apical surface of the follicular cells. As LED lighting continues to expand globally, understanding how different light spectra, intensities, and exposure durations influence thyroid function, particularly during early life stages, remains an important direction for future research.