Potential role of latent toxoplasmosis in inducing thyroid disorders with relevance to autoimmune thyroid disease and interleukin-33 level during pregnancy
Authors: Murad Manal Adil , Eassa Souzan Hussain 
Published: August 2023

Background and Aim: Latent toxoplasmosis is the most frequently occurring parasitic infection worldwide, which causes hormonal and behavioral changes that seriously affect pregnant women. It has also been linked to several autoimmune diseases, including autoimmune thyroid disease (AITD). This study aimed to investigate the association between Toxoplasma gondii seropositivity and thyroid dysfunction, considering the impact of latent toxoplasmosis on the prevalence of maternal AITD and interleukin-33 (IL-33) levels in infected and non-infected pregnant women.

Materials and Methods: We conducted a cross-sectional study on 400 pregnant women aged 15–50 in the 8th–36th gestational week. Toxoplasma status was confirmed by detecting anti-Toxoplasma immunoglobulin (Ig)G antibodies. Recent and past chronic toxoplasmosis status was differentiated using the Toxoplasma IgG avidity test. Free triiodothyronine (FT3), free thyroxine, and thyroid-stimulating hormone (TSH) levels were determined to evaluate thyroid disorders. Antibodies against thyroid peroxidase, thyroglobulin, and TSH receptor were assessed to distinguish patients with autoimmune thyroid disorders from those with other thyroid diseases. We divided the subjects into four groups (Toxo+ and abnormal hormone level, Toxo– and normal hormone level, Toxo+ and normal hormone level, and Toxo– and abnormal hormone level) and evaluated their IL-33 levels to investigate its role during the infection. All the tests were performed using the enzyme-linked immunosorbent assay.

Results: The results showed that (205/400, 51.2%) samples were seropositive for anti-Toxoplasma IgG antibodies. Of these, (25/205, 12.2%) and (180/195, 87.8%) had recent and past chronic infections, respectively. High infection rates were reported among rural dwellers (150/400, 37.5%) and those in their 3rd trimester (110/400, 27.5%). Of the 205 seropositive patients, (131/205, 63.9%) had thyroid disorders, among which (69/205, 33.7%) and (119/205, 58.0%) had abnormal FT3 and TSH hormone levels, respectively. In contrast, out of 195 Toxoplasma seronegative samples, (99/195, 50.8%) had thyroid disorders; (48/195, 24.6%) and (90/195, 46.2%) had abnormal FT3 and TSH hormone levels, respectively. Groups with abnormal FT3 and TSH levels had significantly higher seropositive anti-toxoplasma IgG antibodies (p = 0.01). Women with seropositive anti-Toxoplasma IgG antibodies had a high hypothyroidism rate (115/205, 56.1%) compared with those with seronegative anti-Toxoplasma IgG antibodies (86/195, 44.1%). We found an association between toxoplasmosis and thyroid status (p < 0.05). Out of 400 samples, 85 (85/400, 21.25%) had AITD. Further, (58/205, 28.3%) of women with seropositive anti-toxoplasma IgG antibodies had AITD compared to (27/195, 13.85%) in the seronegative ones. We found a significant association between toxoplasmosis and AITD (p < 0.05). The IL-33 level was highest in the Toxo+ and abnormal hormone level group (210.86 ± 44.39 pg/mL) and lowest in the Toxo-and normal hormone level group (22.27 ± 8.41 pg/mL).

Conclusion: Our results suggest that latent toxoplasmosis was significantly associated with thyroid hormone secretion, which might stimulate the immune system, leading to the development of AITD among pregnant women. Furthermore, the T. gondii seroprevalence was positively correlated with pregnant patients who were rural dwellers and in their 3rd trimester.

Keywords: autoimmune thyroiditis disease, interleukin-33, pregnancy, thyroid hormones, toxoplasmosis.