Mild hypothyroidism improves glucose tolerance in experimental type 2 diabetes

• Improvement in glucose tolerance when mild hypothyroidism coexists with diabetes.
• Improvement in oxidative stress when mild hypothyroidism coexists with diabetes.
• Mild hypothyroidism a possible physiological adaptation in diabetes?.

Background: Co-existence of type 2 diabetes and hypothyroidism is an emerging trend observed in clinical practice. Although the effects of isolated type 2 diabetes and hypothyroidism are well known, there are limited studies addressing the metabolic complications when these two conditions co-exist. The aim of the present study was to assess the interaction between type 2 diabetes and hypothyroidism with respect to glucose tolerance, dyslipidemia and redox balance in a state were these two diseases coexist. Methods: Sixty male Wistar Albino rats were randomised into six groups. Group 1: control, Group 2: overt hypothyroidism, Group 3: mild hypothyroidism, Group 4: type 2 diabetes, Group 5: mild hypothyroidism + type 2 diabetes, Group 6: overt hypothyroidism + type 2 diabetes. Experimental hypothyroidism was created by the administration of propyl-2-thiouracil and type 2 diabetes by feeding rats with 60% fructose (w/w). The duration of the study was 6 weeks. All the parameters were estimated at the start (basal) and the end of the study. Intraperitoneal glucose tolerance test was carried out and area under curve (AUC) calculated to assess the glucose tolerance. Thyroid profile, lipid profile and oxidative stress parameters were also measured. Results: Plasma TSH level was elevated 3-fold in the mild hypothyroid group and 15.2-fold in the overt hypothyroid group in comparison to the control group. Thyroid profile was found to be normal in type 2 diabetic group. There was no significant difference between hypothyroidism and hypothyroidism + diabetes groups with respect to thyroid profile. Among the six groups the degree of glucose intolerance was found to be maximum for overt hypothyroidism + diabetes group, followed by diabetes group and overt hypothyroidism group. An interesting finding was that glucose intolerance was significantly reduced in mild hypothyroidism + diabetes group (increase in AUC: 48.04%) in comparison with isolated diabetes group (increase in AUC: 71.63%). Similar results were obtained with parameters of oxidative stress. Oxidative stress was observed in overt hypothyroidism + diabetes, diabetes, overt hypothyroidism groups with severity decreasing in that order. Coexistence of mild hypothyroidism with diabetes decreased oxidative stress in comparison with isolated diabetes group. There was no statistical difference in lipid profile between mild hypothyroidism + diabetes and isolated diabetes group. Conclusion: Presence of mild hypothyroidism in type 2 diabetes confers a protective effect with respect to glucose tolerance and redox balance whereas presence of overt hypothyroidism in type 2 diabetes has a deleterious effect. The increased incidence of hypothyroidism in diabetes, especially subclinical hypothyroidism, could be a reflection of a physiological attempt by the body to mitigate damage wrought by diabetes.