Abstract
Vitamin D plays an essential role in prevention and treatment of osteoporosis. Thyroid hormones, in addition to vitamin D, significantly contribute to regulation of bone remodeling cycle and health. There is currently no data about a possible connection between vitamin D treatment and the thyroid in the context of osteoporosis. Middle-aged Wistar rats were divided into: sham operated (SO), orchidectomized (Orx), and cholecalciferol-treated orchidectomized (Orx + Vit. D3; 5 µg/kg b.m./day during three weeks) groups (n = 6/group). Concentration of 25(OH)D in serum of the Orx + Vit. D3 group increased 4 and 3.2 times (p < 0.0001) respectively, compared to Orx and SO group. T4, TSH, and calcitonin in serum remained unaltered. Vit. D3 treatment induced changes in thyroid functional morphology that indicate increased utilization of stored colloid and release of thyroid hormones in comparison with hormone synthesis, to maintain hormonal balance. Increased expression of nuclear VDR (p < 0.05) points to direct, TSH independent action of Vit. D on thyrocytes. Strong CYP24A1 immunostaining in C cells suggests its prominent expression in response to Vit. D in this cell subpopulation in orchidectomized rat model of osteoporosis. The indirect effect of Vit. D on bone, through fine regulation of thyroid function, is small.
Highlights
IntroductionCholecalciferol (Vit. D3 ) is a secosteroid hormone that is synthesized endogenously in the epidermis from 7-dehydro-cholesterol, induced by ultraviolet radiation from sunlight both in humans and rodent models [1]
The results on absolute and relative thyroid weight, as well as body mass are summarized in the Table 2
We showed—for the first time—that vitamin D3 treatment of Orx middleaged rats, our model of osteoporosis, changed thyroid morphology in a way that indicates an intensified colloid resorption and hormone release, which was probably compensated by lower hormone synthesis, as circulatory levels of T4 and TSH remained unchanged
Summary
Cholecalciferol (Vit. D3 ) is a secosteroid hormone that is synthesized endogenously in the epidermis from 7-dehydro-cholesterol, induced by ultraviolet radiation from sunlight both in humans and rodent models [1]. D3 ) is a secosteroid hormone that is synthesized endogenously in the epidermis from 7-dehydro-cholesterol, induced by ultraviolet radiation from sunlight both in humans and rodent models [1] It is found in foods (fatty fish, liver, or egg yolks), and is the most common over-the-counter vitamin D supplement, available in a variety of strengths and forms [2]. The main physiological actions of vitamin D are related to regulation of calcium and phosphorus homeostasis, mainly through direct actions of the hormone in the intestine, kidney, and bone, and through feedback inhibition of PTH production in the parathyroid glands [3,4]. In addition to activating enzymes, renal CYP24A1 seems to be mainly responsible for inactivation and degradation of both
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