The present paper describes the annual variations in plasma concentrations and thyroid and hepatic contents of thyroid hormones, as well as the activity of hepatic 5'-T4 ORD (type II) monodeiodinase, in the male lizard, Podarcis sicula, during two consecutive years. Plasma concentration of T4 was low in winter and increased in March, reaching maximal levels in May-June; plasma levels of T3 varied throughout the year, reaching minimum values in December and February. The thyroid content of T3 was high between March and July; the T4 content followed a similar seasonal pattern and showed a minimum in January and a maximum in July. Low hepatic contents of T3 and T4 were found in winter, increasing rapidly after spring to reach maximal values in July-October. Moreover, the maximal activity of hepatic 5'-T4 ORD (type II) monodeiodinase was observed in May and June-July. Minimal activity was detected in winter and in October-November. Plasma TSH levels were high between
The thyroid has been shown to be a target organ for environmental chemicals, specifically endocrine-disrupting contaminants. Reptiles are particularly suitable as contaminant biomonitors due to their persistence in a variety of habitats, wide geographic distribution, longevity, and, in many cases, site fidelity. Methyl thiophanate is a systemic broad-spectrum fungicide used to prevent and control plant diseases caused by various fungi. The aim of this study was to develop an integrated biological model for monitoring the ecotoxic effects of thiophanate-methyl fungicide on the thyroid of the lizard Podarcis sicula. The results of this study indicate that both structural and functional differences in the thyroid gland of the lizard exist in the animals exposed to methyl thiophanate. Structurally, animals exposed to methyl thiophanate showed decreased epithelial cell height; the nuclei of the thyroid cells were small and elongated with dense chromatin and a greatly reduced cytoplasm. The colloid was retracted with few reabsorption vacuoles. Functionally, the same animals exhibited decreased T4 and T3 plasma levels compared to control animals. Methyl thiophanate administration produced statistically significant inhibition on serum thyroid-stimulating hormone levels and this is the mechanism for altering thyroid function. This study highlights how thyroid gland disruption, both structural and functional, in lizard and other nontarget organisms might also have an environmental aetiology.
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