# Influence of Artificial Light at Night on Thyroid Gland Histology in Triturus Newts (Urodela, Salamandridae)

## TL;DR

This study shows that artificial light at night may alter thyroid gland structure and function in newts, potentially affecting their endocrine system.

## Contribution

The study reveals morphological and functional thyroid changes in newts due to artificial light at night, suggesting endocrine disruption.

## Key findings

- No significant differences in thyroid gland volume densities were found between control and light-exposed newts.
- Light-exposed newts showed stronger thyroglobulin signals, indicating possible thyroid hypofunction.
- Artificial light at night may disrupt thyroid gland morphology and secretion in metamorphosed newts.

## Abstract

Artificial light, while beneficial for human activities at night, poses potential health risks to humans and wildlife, particularly affecting amphibians, which are experiencing a global population decline. This study investigates the impact of artificial light at night on the thyroid gland morphology of Triturus ivanbureschi metamorphosed juveniles. Using LED lighting, juveniles were assigned to a control group with dark night conditions and a treatment group maintained under a light intensity of 30 lux for 60 days. Histological analysis revealed no significant differences in the absolute volume densities of the thyroid gland between groups, as well as in the volumes of the follicular epithelium, colloid, and interstitium, although some morphological variations were noted. Control thyroid glands showed uniformity, smaller follicles, and signs of active secretion, while light-exposed glands displayed macrofollicles with a less active state. Immunohistochemical analysis indicated stronger thyroglobulin signals in the light-treatment group, suggesting accumulation at the follicular cell surface and a potential hypofunctional thyroid state. These findings imply that artificial light at night may disrupt endocrine function and affect amphibian thyroid gland morphology and secretion. Further molecular research is needed to understand the physiological implications of these structural changes.

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.