Page 95 - GTM-4-2

Global Translational Medicine Thyroid status in obesity

hormonal pattern in obesity includes reduced free T4 had subclinical hypothyroidism. Thyroid autoantibodies
(fT4) levels and moderately elevated levels of total or free (TPO-Ab and TG-Ab) were found in 17.64% of the obese
T3 (fT3) levels. In addition, the fT3/fT4 ratio has been subjects. Thus, a large body of data in the literature
7
positively associated with waist circumference and BMI in highlights the role of autoimmune processes in obesity-
obese patients. The mechanisms underlying these changes related thyroid glands. However, alternative mechanisms
3
remain controversial. influencing thyroid hormone synthesis remain to be fully
elucidated and warrant further investigation.
Leptin, an adipose tissue hormone, plays an important
role in the regulation of the hypothalamic-pituitary-thyroid The aim of this study was to investigate indices of
axis. It activates this system in response to excess caloric thyroid functional activity in diet-induced visceral obesity
intake. This adaptive response to overnutrition results in a in male and female rats.
moderate increase in blood levels of thyrotropin-releasing
hormone (TRH) and TSH. This is accompanied by a slight 2. Materials and methods
increase in circulating thyroid hormone levels within or 2.1. Animals and diets
slightly above the upper limit of normal. 2
This study was conducted on 2-month-old sexually mature
One theory explaining the relationship between obesity male and female Wistar rats. These rats were bred in-house
and thyroid function is increased deiodinase activity, in a certified vivarium at the Institute of Physiology of the
leading to enhanced conversion of T4 to T3. This may National Academy of Sciences of Belarus. The rats were
2
serve as a compensatory mechanism to counteract fat kept under controlled conditions (12/12 h light/dark cycle,
accumulation by increasing energy expenditure. Another 22 ± 2°C temperature, and 60 – 65% humidity). The male
2
hypothesis suggests that obesity-induced reductions in (n = 27) and female (n = 27) Wistar rats were randomly
TSH receptor expression in the thyroid and T4/T3 receptor divided into two experimental groups: control and high-
expression in adipocytes drive compensatory increases calorie diet (HCD). The control group, consisting of
in TSH and fT3 secretion. In addition, leptin influences 13 male and 14 female rats, received a standard diet (StD;
2
thyroid function by stimulating the central conversion of the normal diet at the vivarium), while the HCD group,
prothyroliberin to TRH and, consequently, the synthesis comprising 14 male and 13 female rats, was given an HCD
of TSH. Leptin also increases the activity of deiodinases. for 16 weeks.
Pro-inflammatory cytokines secreted by adipose tissue – The HCD consisted of StD supplemented with animal
such as tumor necrosis factor-alpha, interleukin-1 (IL)1, fats (lard), consisting of 45% of daily caloric intake, along
and IL-6 – may also contribute to thyroid dysfunction by with a 10% fructose solution provided ad libitum in place
inhibiting sodium/iodide symporter Na+/I- symporter of water. The caloric intake was 150 kcal/day for StD and
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(NIS) mRNA expression and iodide uptake. 2 228 kcal/day for HCD.
Epidemiological studies suggest that the risk of obesity This study was approved by the Bioethics Committee of
is associated with elevated TSH levels and reduced the Institute of Physiology, National Academy of Sciences
concentrations of the free T4 and T3 hormones. These of Belarus (Protocol No.1, January 22, 2021; Protocol No.2,
4,5
data suggest that reduced levels of circulating thyroid February 2, 2022) and conducted in accordance with the
hormones are a predictor of obesity. Hypothyroidism guidelines set forth by the European Convention for the
and thyroperoxidase antibodies (TPO-Ab) are highly Protection of Vertebrate Animals (ETS No. 123).
prevalent in people with excess fat mass. A meta-analysis
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of 22 studies showed that obesity is associated with an Euthanasia was performed through decapitation under
increased risk of clinical and subclinical hypothyroidism. sodium thiopental anesthesia. Female rats were euthanized
6
Symptoms of hypothyroidism, including reduced physical during the diestrus phase of the estrous cycle, determined
activity, fatigue, sleepiness, memory loss, cold intolerance, by the type of cells present in vaginal swabs. 9
weight gain, and constipation, often develop gradually. Body weight of the rats was measured using a weighing
Another meta-analysis found that obesity was significantly scale (Saturn ST-KS7230, China). After euthanasia, the
associated with Hashimoto’s thyroiditis, but not with visceral fat was collected and weighed on a laboratory
Graves’ disease. Weight gain correlated with TPO-Ab weighing scale (Scout Pro, China). For male rats, the
positivity but not with thyroglobulin antibodies (TG- visceral fat mass included paranephral and epididymal
Ab). However, some studies have reported associations fat deposits, while for female rats, it included paranephral
6
between subclinical hypothyroidism and both TPO-Ab and periovarian fat deposits. The mass coefficient (MC) of
and TG-Ab. A survey of 2,505 subjects showed that 11.5% the visceral fat was calculated using the following formula:
of the subjects (289/2505) were obese, of whom 165 (57%) (Visceral fat mass/body weight) ×100%.

Volume 4 Issue 2 (2025) 87 doi: 10.36922/GTM025080020