Free Triyodothyronine - Free Thyroxine Ratio in Euthyroid Obesity Patients
Keywords:Triiodothyronine, thyroxine, obese
Obesity is the condition of body mass index (BMI) ≥ 30 kg/m² as a result of excessive increase in body fat mass. In recent years, obesity rates have more than doubled, and about a third of the world's population is now classified as overweight or obese. Obesity can affect thyroid hormones in addition to the role of thyroid hormones in regulating body weight. Therefore, by defining the relationship between obesity and thyroid hormones, the risk factors of many diseases can be controlled.
The aim of this study is to investigate the fT₃-sT₄ ratio in obese patients without known thyroid disease and normal thyroid function tests.
Cases examined retrospectively were divided into 2 groups as obese and control (non-obese) groups. In the obese group, 58 patients (34 women, 24 men) and 60 healthy subjects (39 women, 21 men), a total of 118 patients aged 18-64 years with biochemical data were included in the study.
The thyroid function tests of the groups were compared. The fT₃ level was found to be 3.9 (3.84-3.99) pg/ml in the obese group, and 2.88 (2.74-2.98) pg/ml in the control group (p<0.001). The fT₄ level was 0.92 (0.91-0.95) ng/dl in the obese group, and the fT₄ level was 1.52 (1.4-1.63) ng/dl in the control group, which was statistically significant (p<0.001). TSH level was 3.79 (3.54-3.98) mIU/mL in the obese group and 1.78 (1.48-2.15) mIU/mL in the control group (p<0.001). The fT₃/sT₄ ratio was 4.22 (4.03-4.35) in the obese group and 1.9 (1.76-2.1) in the control group (p<0.001).
Given that obesity is characterized by an increase in both muscle mass and sT₃ levels, it may well be that a higher muscle mass is responsible for higher type II iodothyronine deiodinase, thereby increasing fT₃ production and circulating hormone levels. Recent studies have found that serum sT₃ levels increase in obese patients. In euthyroid individuals, the fT₃ and fT₃-sT₄ ratios were positively correlated with BMI, waist circumference, and body fat percentage. In our study, sT₃ level and fT₃-sT₄ ratio were found to be higher in the obese group compared to the control group, in line with the literature.
In our study, we found that the fT₃-sT₄ ratio is metabolically important and associated with obesity in cases without known thyroid disease and whose thyroid function tests are completely euthyroid.
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