Association between Altered Thyroid Function and Prediabetes in Diet-Induced Prediabetic Male Sprague Dawley Rats

1. Introduction

Type 2 diabetes mellitus (T2DM) is regarded as a metabolic disorder which occurs due to impaired insulin action on target tissues, thus resulting in chronically elevated blood glucose concentrations [1]. There are numerous long-term metabolic complications associated with T2DM [2]. However, several studies have shown that the onset of T2DM complications occurs during the prediabetic state [3,4]. Prediabetes (PD) is regarded as a state of moderate hyperglycaemia during which glycaemic indicators such as impaired fasting glucose, impaired glucose tolerance, elevated glycated haemoglobin (HbA1c), as well as homeostasis model assessment (HOMA-IR) levels are elevated above the normal range but are below the threshold for diagnosis of T2DM [5,6]. In addition, studies have shown that the functions of some metabolic hormones such as insulin, ghrelin, cortisol and aldosterone are impaired in the prediabetic state [7,8,9].

Thyroid hormones, triiodothyronine (T3) and thyroxine (T4), are regulatory hormones which are primarily associated with metabolic function and energy expenditure [10]. These hormones, along with thyroid peroxidase (TPO), are vital components of glucose homeostasis while the dysregulation of thyroid hormones has been found to correlate with reduced insulin sensitivity, metabolic dysfunction and T2DM [11,12]. Plasma creatine and insulin levels have also been used to assess changes in metabolic functions which are associated with irregular thyroid hormone production [13]. Studies have indicated that individuals diagnosed with T2DM are at a higher risk of developing thyroid disorders [14,15]. Both hyperthyroidism and hypothyroidism have been associated with diabetes mellitus, with the latter being the more prevalent diagnosis in T2DM [16]. Studies in our laboratory using a diet-induced prediabetic animal model have shown that the genesis of various endocrine disorders occurs during the prediabetic phase [8,9,17,18]. However, the association between prediabetes and thyroid hormone function remains unknown. In this study, using a diet-induced prediabetic rat model, we sought to determine the changes in some selected markers of thyroid function in prediabetes.

2. Materials and Methods

3. Results

4. Discussion

The association between T2DM and thyroid dysfunction has been extensively studied[13]. Several studies have not only identified prediabetes as the period that precedes the onset of overt type 2 diabetes but also a period where numerous complications associated with T2DM begin [3,19]. Thyroid hormones play a regulatory role in glucose homeostasis, metabolic function and energy expenditure [10]. There have been several studies that have investigated changes in thyroid function during T2DM [11,12]. However, no studies have been conducted to investigate thyroid function during the prediabetic phase. In our laboratory, the HFHC diet-induced prediabetic rat model was used to mimic the human prediabetic condition [17]. Using this animal model, we investigated thyroid function during the prediabetic state.

The thyroid hormones, triiodothyronine (T3) and thyroxine (T4) have an imperative function in both metabolic and protein synthesis pathways [20]. Previous studies have shown that individuals diagnosed with T2DM often present with supraphysiological concentrations of insulin, coupled with a dysregulation in the plasma levels of thyroid hormones [21]. The synthesis of thyroid hormones requires the cumulative effort of the thyroid gland, regulated biochemical signaling cascades and sufficient dietary iodine intake [22]. The hypothalamic-pituitary-thyroid axis (HPT-axis) is stimulated to regulate the excitatory and inhibitory signals which trigger the production of the thyroid hormones. Thyroid stimulating hormone (TSH), released from the anterior pituitary gland, is the primary stimulator of thyroid hormone synthesis [23]. The release of TSH is stimulated by thyroid releasing hormone (TRH) from the hypothalamus, both of which are regulated via the negative feedback loops that acts on the HPT axis [24]. Studies have shown that impairment of these feedback loops result in increased release of both T3 and T4 hormone production simultaneously [25,26]. In this study, the T3 levels were found to be elevated in the prediabetic group whereas the T4 levels were reduced. These findings were supported by previous literature which showed that a subsequent increase in T3 hormone production coincided with a reduction in T4 production [27]. This imbalance in hormone synthesis is suspected to arise from the thyroid gland itself rather than the HPT axis [27]. The synthesis of thyroid hormones requires a multi-step process along with vital metabolic and cellular constituents. This intricate process is initiated by the binding of TSH to the TSH-receptor on the surface of the follicular cell within the thyroid gland. Once the receptor-ligand complex has formed, an intracellular signalling cascade is stimulated thus, leading to the synthesis of T3 and T4 [28]. The major constituents of thyroid hormones are tyrosine and iodine. A key enzyme in the synthesis of thyroid hormones is thyroid peroxidase (TPO) which has multiple functions in the synthesis pathway. It is responsible for the formation of iodine by oxidizing the iodide ions as well as initiating the formation of covalent bonds between the iodide and the thyroglobulin [29]. This covalent bonding ultimately leads to the synthesis of monoiodotryrosine (MIT) and diiodotyrosine (DIT), depending on their single or double iodinated state [30]. TPO then regulates the generation of T4 by coupling two DIT residues or T3 by coupling a DIT with a MIT.

Studies have identified T4 as the more abundant thyroid hormone as TPO is predisposed to promote the coupling of DIT-to-DIT residues rather than the alternative DIT to MIT [31]. Thus, a reduction in T4 hormone production may be attributed to the simultaneous reduction in TPO enzyme synthesis or function. Therefore, the results of this study corroborated with that of previous literature as the plasma T4 levels were significantly reduced in the prediabetic group along with an increase in TPO antibody production. TPO enzyme function is disrupted by the production of TPO antibodies which are synthesized by the lymphocytes of the autoimmune system [32]. Thyrocyte destruction precedes the action of TPO antibodies as they exert an intracellular inhibitory effect on the TPO enzymes and their associated signalling cascade [33]. TPO antibodies competitively inhibit the action of TPO enzyme activity by forming conformational isotopes of the enzymes which impede its function in the thyroid hormone synthesis pathway. These antibodies also attribute to the fixation of complement which results in the destruction of the thyroid gland [34]. In this study, we found elevated levels of TPO antibody production in the prediabetic group as compared to the non-diabetic group. A previous study has shown that there is dysregulation of immune function during the prediabetic state leading to an elevation of lympohycytes [18]. We therefore speculate that the reduction in TPO enzyme activity may be attributed to the increase in TPO antibody production, thus the thyroid hormone synthesis pathways favored the deionization of T4 to T3 in response to the diminished hormone production.

The reduction in the T4 hormone concentration leads to the reduced function of the thyroid gland [35]. This leads to a reduction in circulating T4 levels resulting in diminished biological function [36]. One such physiological function is that of protein synthesis. In a previous study, elevated levels of thyroid hormone were found to correlate with the destruction of protein stores from skeletal muscle [37]. Thus, an inference can be made regarding the reduced creatinine levels which form as a by-product of protein degradation in the prediabetic group as the reduction in T4 would subsequently enhance the synthesis and degradation of proteins. A previously published study showed no significant changes in glomerular filtration during the prediabetic state thus eliminating hyperfiltration as a possible cause for the reduction in creatinine levels [38]. The reduced plasma creatinine observed in the prediabetic group may therefore be an indicator of reduced protein synthesis, preceded by the reduction in thyroid hormone activity [39].

Another imperative function of thyroid hormones is to facilitate the breakdown of insulin [21]. Previous studies have found a correlation between insulin resistance and thyroid disorders. Thyroid hormones have been found to impede the action of insulin whilst stimulating the hepatic gluconeogenesis [40,41]. The increase in plasma insulin observed in the prediabetic group may be due to the diminished activity of the circulating thyroid hormones. Thus, there is hyperinsulinemia as well as increased insulin half-life which promotes insulin resistance [12]. This may be a compensatory mechanism that arises due to the cells’ failure to respond to insulin leading to a fall in T4 concentrations thus ensuring higher insulin concentrations. Also, studies have found a positive correlation between elevated T3 levels and insulin resistance [42]. T3 was found to be actively involved in the regulation of metabolic transcription factors which are attributed to insulin resistance [43,44]. In this study, we found elevated T3 levels in the prediabetic state. This suggests that thyroid dysfunction in the prediabetic state may contribute to the progression of prediabetes to overt type 2 diabetes. Since T4 is the more abundant thyroid hormone, a reduction in its plasma concentration may suggest the onset of a thyroid disorder. On the other hand, the increase in the biologically active T3 hormone, may be due to the impediment of the TPO enzyme leading to the over activity of the deiodinase system resulting in an increase in T3 production [45]. The interaction between the concentration of the thyroid hormones along with their downstream enzymes, metabolic and biosynthetic pathways, attests to the importance of the thyroid and the consequences of its dysfunction.

Moreover, in this study, there was increased glycated haemoglobin (HbA1c) in correlation with increased T3 and decreased T4 concentration in prediabetic rats while this was not observed in non-prediabetic rats. One of the characteristics of prediabetes is elevated HbA1c which has been associated with insulin resistance. As discussed above, the increase in T3 or decrease in T4 production may be a compensatory mechanism to alleviate insulin resistance in prediabetic condition. Therefore, we may suggest that the compensatory mechanism engendered the observed positive correlation of HbA1c with T3 and negative correlation of HbA1c with T4 in the prediabetic rats. However, the limitation of this study is the absence of molecular and cellular changes that occur in the thyroid gland following prediabetes, therefore, future studies are needed to be carried out in these areas.

5. Conclusions

In conclusion, previous studies had shown the associations between thyroid hormone dysregulation in overt type 2 diabetes. This study has shown for the first time, using a diet-induced prediabetic animal model, that these thyroid hormone dysregulations also occur in prediabetes, the condition that often precedes the onset of type 2 diabetes. In this study, there was a positive correlation between diet-induced prediabetes and thyroid dysfunction. This is evidenced by the notable increase in triiodothyronine (T3) and subsequent decrease in thyroxine (T4) hormone concentrations as HbA1c increase in the prediabetic state. This observed dysregulation of thyroid hormones was further evidenced by the increased production of TPO antibodies. Taken together, the finding of this study suggest that thyroid hormone homeostasis is disrupted during prediabetes and this warrants more research on the thyroid hormone synthesis pathway within the thyroid gland.