Relationship between thyrotropin and body mass index in euthyroid subjects.

BACKGROUND

Several studies have reported that thyrotropin (TSH) levels are slightly increased in obese subjects. On the contrary, other studies have provided no evidence for an association between thyroid status and body mass index (BMI).

AIM: Our aim has been to evaluate the relationship between TSH and BMI in a group of subjects with obesity, overweight and normal weight with serum TSH levels within the reference range, excluding subjects with hypothyroidism and hyperthyroidism.

SUBJECTS AND

METHODS

We evaluated a cohort of 778 euthyroid subjects (488 women, mean age 58.5 ± 15.9 yr) with TSH values between 0.4 and 5.0 mU/l. There were 321 subjects with obesity (BMI ≥ 30 kg/m²), 319 with overweight (25-29.9 kg/m²) and 138 with normal weight (18.5-24.9 kg/m²).

RESULTS

Serum TSH levels significantly increased with weight category in studied subjects (normal weight, median [interquartile range], 1.24 [0.94-1.72]; overweight, 1.31 [1.00-1.93]; obesity, 1.66 [1.15-2.44] mU/l; P<0.001). Free thyroxine (T4) was not statistically different in subjects within the 3 weight categories. A significant correlation was found between TSH and BMI (r=0.217, P<0.001). Subjects with serum TSH levels in the upper tertile (1.84-5.0 mU/l) exhibited a BMI value (30.8 [26.9-34.9] kg/m²) significantly higher (P<0.001) than that found in subjects with TSH levels in the middle (1.13-1.83 mU/l, 28.8 [25.8-32.3] kg/m²) or in the lower tertile (0.4-1.12 mU/l, 27.4 [25.4-30.6] kg/m²). There were no differences in BMI in patients classified according to tertiles of free T4. However, when studying a subgroup of patients with negative thyroid autoimmunity (n=375), we could not observe any significant difference in TSH levels between obese (n=106, 1.24 [0.90-1.81] mU/l) and non-obese subjects (n=269, 1.23 [0.90-1.80] mU/l). No correlation was found between TSH and BMI in this subgroup of subjects with negative thyroid autoimmunity (r=-0.035, NS).

CONCLUSION

These results suggest that the relationship between TSH and BMI observed in euthyroid subjects is lost when subjects with negative autoimmunity are selected.