Biochemical and Hormonal Alterations in the Rat Thyroid Gland Following Chronic Ultraviolet-A Exposure: A Duration-Dependent Study

Abstract

Ultraviolet-A (UVA) radiation, the most prevalent component of solar UV reaching the earth's surface, is known to induce oxidative stress and cellular damage in various tissues. However, its specific effects on endocrine organs, particularly the thyroid gland, remain inadequately characterized. This study aimed to evaluate the biochemical and hormonal changes in the thyroid gland of rats following chronic exposure to UVA radiation for different durations. Twenty adult male albino rats (200-250 g) were divided into four groups (n=5 each): a control group (no exposure) and three experimental groups exposed to UVA radiation (8 hours/day) for 10, 15, and 25 days, respectively. Thyroid tissue levels of triiodothyronine (T3), thyroxine (T4), thyroid-stimulating hormone (TSH), and the antioxidant enzyme reduced glutathione (GSH) were measured. Chronic UVA exposure significantly altered thyroid hormone profiles. T3 levels initially increased at 10 days (3.65±0.92 vs. control 2.6±0.25) and 15 days (3.62±0.29), then declined towards near-normal levels by 25 days (2.9±0.53). T4 showed a non-significant fluctuation, while TSH exhibited a dramatic and sustained reduction from 0.8±0.14 (control) to near-undetectable levels (0.016±0 to 0.01±0) in all exposed groups. GSH levels varied, showing a non-significant decrease at 10 days (54.63±1.31), followed by an increase at 15 and 25 days (59.2±3.78 and 58.01±2.28, respectively). Chronic UVA radiation exposure induces duration-dependent disruption of thyroid hormonal homeostasis, characterized by marked TSH suppression and complex alterations in T3 and T4, accompanied by changes in antioxidant enzyme levels. These biochemical findings complement previously reported histological damage (Qabeeli et al., 2026) and suggest that UVA radiation may impair the hypothalamic-pituitary-thyroid axis