Free triiodothyronine plasma concentrations are positively associated with insulin secretion in euthyroid individuals

Abstract

Background: Thyroid hormones (TH) may influence glucose metabolism. Hyperthyroid subjects have higher insulin secretion rates when compared with euthyroid individuals. Objective: To evaluate the association between TH concentrations and insulin secretion in euthyroid, healthy Pima Indian adults (nZ55, 29G7 years, females/males 36/19) with normal glucose tolerance (NGT) admitted to a Clinical Research Unit. Methods: TSH, free thyroxine (FT 4 ), 3,5,30 -L-tri-iodothyronine (FT 3 ), and fasting plasma insulin (FPI) concentrations were measured in fasting plasma samples, percentage of body fat (%BF) by dual energy x-ray absorptiometry (DXA), acute insulin response (AIR), and incremental area under the curve (AUC) of insulin in response to a 25 g intravenous glucose tolerance test (IVGTT) and 75 g oral glucose tolerance test (OGTT) respectively and insulin action (M) during an euglycemic clamp. Results: FT 3 concentrations were associated with FPI, AIR, and insulin AUC both before (rZ0.33, PZ0.01; rZ0.29, PZ0.03; and rZ0.35, PZ0.008 respectively) and after adjustment for age, sex, %BF, glucose (fasting concentrations or glucose AUC), and M (bZ0.09, PZ0.01; bZ0.16, PZ0.03; and bZ0.24, PZ0.0007 respectively). No associations were found for TSH or FT 4 . Conclusion: FT 3 was associated with several measurements of insulin secretion in euthyroid individuals with NGT. T 3 concentrations may play a role in the regulation of insulin secretion.

Raised serum TSH levels in patients with morbid obesity: is it enough to diagnose subclinical hypothyroidism?

“…Unlike TSH, data regarding the circulating levels of free thyroid hormones are discrepant between different studies, which reported either increased or decreased serum concentrations of FT 3 (11,12,14), with normal or decreased FT 4 /FT 3 ratios (12,14). Discrepancy is most likely due to the inclusion of patients with different degrees of obesity, as assessed by BMI, or different fasting plasma insulin concentrations, which were recently demonstrated to be positively related to FT 3 levels, independently from plasma glucose concentration (20).…”

Indices of Central and Peripheral Obesity; Anthropometric Measurements and Laboratory Parameters of Metabolic Syndrome and Thyroid Function

“…Additionally, Bilgin and Pirgon (32) reported that obese adolescents with steatohepatitis had elevated values for fT3/fT4 ratio. Ortega et al (33) suggested that fT3 was related with insulin secretion in euthyroid subjects, and T3 levels may be involved in the regulation of insulin secretion. In another study, it was proposed that alterations in thyroid hormones could be the result rather than the source of the increasing weight (34).…”

Serum vaspin levels in hypothyroid patients

“…In some recent studies, thyroid dysfunction (23) and even TSH within the normal range (19,24,25) were reported to be associated with insulin resistance and components of metabolic syndrome. By contrast, some other studies failed to detect a significant association between TSH and insulin resistance (19,21,22,26). It is difficult to prove a casual relationship between insulin resistance and thyroid hormones as both are influenced by weight status.…”

“…The relationship between hypothyroidism, obesity and insulin resistance is of clinical interest (19)(20)(21)(22). To elucidate the link between thyroid disorders and adipose tissue, various adipocytokines were previously studied (9-11), in which the most promising one was leptin.…”