Overt T2DM is a complex metabolic condition involving various endocrine hormone interactions. Since hormones tend to fluctuate with ageing, the findings for this study are adjusted for ageing by only including participants between 25–45 years of age. It is unclear if the hormone dysregulations observed in T2DM will be seen in individuals with pre-diabetes between 25 – 45 years. Hence, this study highlights the various hormones linked with T2DM in persons with pre-diabetes.

C-peptide is a parameter for insulin sensitivity and insulin production. Therefore, in Fig. 1, it is unsurprising that high c-peptide levels were detected in the pre-diabetes participants. Another study showed increased concentrations of C-peptide detected in insulin-resistant individuals recently diagnosed with T2DM, which supports the study’s C-peptide outcome in subjects with pre-diabetes [23]. In Table 3, c-peptide positively correlates with HbA1c, suggesting that c-peptide contributes to glycemic control. We speculate that C-peptides remain elevated chronically during pre-diabetes to compensate for insulin resistance. If insulin resistance is unattended during pre-diabetes, then the C-peptide levels will continue to rise and reach the levels observed in T2DM. Interestingly, in Table 2, the hormone insulin levels are higher but not statistically significant in pre-diabetes participants. This may indicate that the level of insulin resistance is still moderate in persons with pre-diabetes.

The incretin, glucose-dependent insulinotropic polypeptide (GIP) levels shown in Fig. 1 are significantly lower in the pre-diabetes group compared to the non-pre-diabetes group. The GIP hormone is critical in maintaining the glucose homeostatic range by stimulating beta-cell proliferation and insulin secretion [24]. Comparable results have been observed in studies using participants with T2DM, where the effectiveness of GIP is almost completely lost [25, 26]. A meta-analysis study on individuals with T2DM showed that higher glycated haemoglobin (HbA1c) concentrations and ageing were linked with reduced GIP responses [27]. However, the results of this study reveal that the decrease in GIP levels begins at the pre-diabetic stage. We speculate that sustained low levels of GIP may be necessary to trigger the onset of hyperglycaemia and, in turn, T2DM. During the pre-diabetes phase, the low levels of GIP may be compensated by another incretin hormone known as GLP-1 to keep glucose levels below the T2DM diagnosis threshold.

The concentrations of GLP-1 in Fig. 1 remained relatively the same between the two groups, as no statistical significance was observed. A prospective study by Hoffmann et al. (2023) also detected no changes in GLP-1 levels at the pre-diabetes state, but a decline in GLP-1 concentrations was seen in patients progressing to diabetes [28]. Therefore, it is plausible that the imbalance of GLP-1 may be a triggering point for the development of T2DM. Therefore, a longitudinal study is warranted to investigate the role of both GIP and GLP-1 in the progression from pre-diabetes to T2DM. A clear understanding of their roles in developing pre-diabetes and its passage to T2DM can play a pivotal role in introducing new therapeutic medications for pre-diabetes patients.

The cortisol concentration in Fig. 1 is significantly higher in individuals with pre-diabetes when compared to those in the non-pre-diabetes group. In Table 3, a positive correlation between cortisol and HbA1c was determined. Elevated cortisol levels have been shown to promote disturbance in glucose homeostasis by inducing visceral fat accumulation, reducing insulin sensitivity in skeletal muscles, and causing lipolysis. The reduction in insulin sensitivity may be exacerbated by the significantly lower levels of the hormone adiponectin. Studies have linked the decrease of adiponectin with insulin resistance and T2DM [29, 30]. Moreover, dysregulation of adiponectin has been observed in non-obese individuals, indicating its potential utility in studies where BMI data is unavailable. [31]. Therefore, adiponectin may be one of the critical causes and markers of interest that promote glycaemic dysregulation, resulting in pre-diabetes and T2DM if left unattended. However, the adverse complications of high cortisol and low adiponectin levels seen in pre-diabetes may be attenuated by the significantly elevated adipsin levels. The hormone adipsin is responsible for sustaining adipose tissue homeostasis and boosts insulin secretion in response to glucose [32]. Hence, adipsin is associated with protection from T2DM and may be one of the key compensatory mechanisms that delay pre-diabetes progression to overt diabetes. However, a longitudinal study that will adjust for the confounding variable, body weight, and trace adipsin levels from pre-diabetes to T2DM is necessary to have a conclusive statement.

The role of sex hormones in T2DM is well-documented [33,34,35]. Their role in pre-diabetes remains limited. The risk of developing diabetes is more significant in men with low testosterone [36]. In females, however, high testosterone concentrations were associated with an elevated risk of diabetes onset [37]. In Table 3, this study observed no significant changes in testosterone levels in the male groups. This suggests that in males aged 25–45 with pre-diabetes, the testosterone levels are not yet dysregulated and may decrease upon the onset of overt diabetes. This also suggests that the leading cause of testosterone drop observed in other studies may be ageing since testosterone levels are known to decrease with age [38]. In females, Table 3 results show that testosterone was higher in participants with pre-diabetes, which supports other literature findings that indicate a higher risk of diabetes in females with high testosterone levels. The other sex hormone investigated is estradiol. Estradiol has a protective impact on the insulin β cells. Compared to the control group, females with pre-diabetes have higher estradiol levels. This may explain why the proportion of females with pre-diabetes is less when compared to males. There is literature evidence showing an increased incidence of diabetes in females who have reached menopause. Estradiol deficiency occurs after menopause, increasing the risk of T2DM onset [39]. Therefore, maintaining the homeostatic range of estradiol may be part of the compensatory mechanisms responsible for preventing T2DM and opens the door for designing new therapeutic targets [40].

Triiodothyronine (T3) and tetraiodothyronine (T4) levels were significantly elevated in the pre-diabetes group and correlated with HbA1c levels, indicating a proportional relationship between thyroid hormones and pre-diabetes onset. In literature, both hypothyroidism and hyperthyroidism have been shown to be associated with T2DM [41]. Hyperthyroidism has been reported to worsen glycaemic control in patients with T2DM. Therefore, it can be one of the essential hormones responsible for the progression from pre-diabetes to T2DM over time [42]. Females are more susceptible to developing thyroid dysfunction than males [43]. The study observed the same trend for women with pre-diabetes in Table 3. However, the understanding of the association remains unclear and necessitates further investigation into the pre-diabetes condition. An intervention study targeting hyperthyroidism in women can be done on participants with diabetes and pre-diabetes to provide empirical evidence that will improve individualised care and management of diabetes.

The findings, although not yet conclusive, raise a question about possible alternative treatments to metformin, such as hormone replacement medications and GIP/GLP-1 agonists, which could be administered in persons with pre-diabetes. For example, a study by Iskra Bitoska et al. (2016) showed that hormone replacement therapy improves insulin sensitivity in women with T2DM [44]. A dual GIP/GLP-1 receptor co-agonist, Tirzepatide, has been sanctioned for managing T2DM. It is a modified acylated peptide that stimulates GIP and GLP-1 receptors [45]. These receptors play pivotal roles in insulin secretion and are also present in areas of the brain responsible for regulating food intake. These interventions may prove beneficial in the management of pre-diabetes; thus, preclinical trials are warranted.