Polystyrene Microplastics-Induced Thyroid Dysfunction in Mice: A Study of Gene Expression, Oxidative Stress, and Histopathological Changes

Abstract

Background: Polystyrene microplastics (PS-MPs) are pervasive pollutants impacting animals across ecosystems, including livestock and wildlife, through contaminated food, water, and air. MPs may disrupt endocrine function, particularly affecting the thyroid gland, which is essential for metabolism and development.

Objectives: This study investigates the effects of PS-MPs on thyroid function in mice, offering insights relevant to veterinary care by examining changes in gene expression and biochemical markers.

Methods: PS-MPs of 5 µm diameter were prepared in distilled water after probe sonication. Sixty male Swiss albino mice were divided into three groups: a control group and two treatment groups receiving 0.1 mg and 0.2 mg PS-MPs via oral gavage for 28 days. Mice were anesthetised, and thyroid tissues were collected for histopathological, biochemical, and gene expression analyses. Biochemical tests included catalase, superoxide dismutase, reactive oxygen species, and hormone levels. Histopathology and gene expression (TSHR and TPO) of thyroid-related genes were examined to assess PS-MPs induced effects.

Results: Exposure to PS-MPs in mice led to significant increases in calcium, thyroxin, free T3, free T4, ALP, AST, ALT, and amylase levels, alongside elevated oxidative stress markers. Conversely, the levels of TSH, calcitonin, magnesium and phosphate decreased. Histopathological analysis showed abnormal thyroid follicle development, decrease parafollicular cells, with colloid loss, haemorrhage, and necrosis. Gene expression analysis revealed a marked reduction in TSHR and TPO levels in PS-MPs treated groups, indicating thyroid dysfunction. These findings highlight the profound impact of PS-MPs on thyroid gland function in mice.

Conclusion: These findings underscore the potential risks that PS-MPs pose to thyroid health, with potential consequences for other veterinary species. As environmental contamination rises, veterinarians may encounter more endocrine disorders linked to PS-MPs, emphasising the need for further research and preventive measures.

Keywords: adverse effects; mice; microplastics; thyroid function.

FIGURE 1

Impact of different concentrations of PS‐MPs on the thyroid gland in mice. The figure presents histological analysis at various magnifications (10×, 40×, and 100×) with H&E staining and scale bars of 80 µm, 20 µm, and 8 µm, respectively. Control Group (C/a, C/b, C/c): Panels show normal thyroid morphology with well‐formed thyroid follicles (TH) filled with colloid (*). The follicular epithelium displays intact cellular junctions (black arrows in C/c), and parafollicular cells are densely populated (yellow circles). T1 Group (T1/a, T1/b, T1/c; 0.1 mg/0.5 mL PS‐MPs): Displays show altered thyroid follicles (TH), including haemorrhage, presence of RBCs, necrotic areas (NA), disrupted cellular junctions (yellow arrows in T1/c), and decreased parafollicular cell density (yellow circles). Colloid depletion is visible (**). T2 Group (T2/a, T2/b, T2/c; 0.2 mg/1 mL PS‐MPs): Similar abnormalities to T1, including more extensive colloid loss (**), disrupted follicular structure, and reduced parafollicular cell presence (yellow circles).

FIGURE 2

Histograms of thyroid follicle dimensions across groups. (A) Follicle length distribution: Histogram showing the distribution of follicle lengths (µm) in the thyroid glands of mice from the Control group, T1 (0.1 mg/0.5 mL PS‐MPs), and T2 (0.2 mg/1 mL PS‐MPs). A clear shift toward larger follicle lengths is observed in the treated groups, particularly in T2, indicating follicular hypertrophy due to PS‐MP exposure. (B) Follicle width distribution: Histogram illustrating the distribution of follicle widths (µm) across the same experimental groups. Similar to follicle length, an increase in follicle width is evident in the treated groups compared to control, further supporting morphological changes associated with PS‐MPs exposure.

FIGURE 3

This figure illustrates the impact of PS‐MPs exposure on the expression of the TSHR gene in mice. Groups T1 and T2, exposed to MPs, exhibit a significant reduction in TSHR gene expression levels compared to the untreated control group (C). Statistical analysis (p < 0.05) reveals significant differences (***) between the treatment groups and the control group, as well as between the treatment groups themselves. These findings highlight the detrimental effects of MP exposure on TSHR gene expression.

FIGURE 4

This figure illustrates the impact of PS‐MPs exposure on the expression of the TPO gene in mice. Groups T1 and T2, exposed to PS‐MPs, show a marked decrease in TPO gene expression levels compared to the untreated control group (C). Statistical analysis (p < 0.05) confirms significant differences (***) between the treatment groups and the control group, as well as between the treatment groups themselves. This underscores the harmful effects of PS‐MPs exposure on TPO gene expression.