Abstract
The aim of the present study was to determine and elaborate on all changes in old-aged (OA) versus young-aged (YA) rat thyroids by using stereological, ultrastructural, hormonal, and gene expression analyses. We used 4- and 24-month-old male Wistar rats in our evaluation, presenting all changes in comparison with YA rats. Results showed that the thyroid parenchyma was characterized by higher absolute volumes of the gland, colloid, epithelium, and interstitium by 135, 135, 140, and 142% (p < 0.05) respectively, while the relative volumes of colloid and glands were unchanged. Ultrastructural analysis revealed less active glands, with smaller amounts of lysosomes, thyroglobulin (Tg) granules, and microvilli in the luminal colloid. Optical density values for thyroid peroxidase (TPO), Tg, and vascular-endothelial growth factor immunostaining remained unchanged; however, TPO and Tg exhibited visually stronger expression in small active follicles. Thyroxine (T4)-Tg, the relative intensity of fluorescence (RIF), serum T4, and the sodium-iodide symporter immunohistochemical and gene expressions decreased by 20, 40, 29, and 31% (p < 0.05), respectively, in OA thyroids. Pituitary thyroid-stimulating hormone (TSH) RIF increased by 44% (p < 0.05), but the TSH serum concentration remained unchanged. In conclusion, the obtained results indicate depression of the thyroid gland synthetic and secretory capacity with advanced age.
Highlights
Aging is characterized by a progressive loss of physiological integrity at the whole-body level
Stereological analysis demonstrated that the absolute volume of the thyroid gland, colloid, interstitium, and epithelium in the OA group increased by 135, 135, 140, and 142% ( p < 0.05; Figs. 1e–1h) respectively, in comparison with the values obtained for YA
We identified changes that included an increment in the absolute thyroid volume, bigger inactive follicles, stronger thyroid peroxidase (TPO) and Tg expression in smaller active follicles, downregulation of NIS IHC and gene expressions, upregulation of Tg, Cat, and Sod1, lower capacity to produce T4, a lower serum T4 level, and an increase in pituitary thyroid-stimulating hormone (TSH) relative intensity of fluorescence (RIF), all in comparison with YA rats
Summary
Aging is characterized by a progressive loss of physiological integrity at the whole-body level. It leads to a decline in endocrine function inducing somatopause, andropause, and menopause (Chahal & Drake, 2007; Ajdžanović et al, 2018). The thyroid axis is characterized by normal thyroxine (T4) and normal or slightly elevated thyroid-stimulating hormone (TSH) levels (Barbesino, 2019). The situation is different, showing normal TSH and lower serum T4 concentrations with advanced age (Donda & Lemarchand-Beraud, 1989; Silvestri et al, 2008). Structural parameters of the thyroid parenchyma mostly correlate with the activity of the thyroid hormone generating system.
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