Significant inverse relationship between serum free T4 concentration and body mass index in euthyroid subjects: differences between smokers and nonsmokers.
OBJECTIVE
There are conflicting data regarding the relationship between thyroid function and body mass index (BMI) in euthyroid subjects, and it is uncertain whether tobacco smoking modifies this relationship. The objective of this study was to examine the relationships between thyroid function, BMI and smoking in euthyroid subjects.


METHODS
Linear regression models were used to examine the relationships between serum free T4, serum TSH, BMI and smoking in a cross-sectional, community-based sample of 1853 euthyroid subjects in Busselton, Western Australia.


RESULTS
There was a significant negative relationship between free T4 and BMI: after adjustment for age and sex, each 1 pmol/l increase in free T4 was associated with a decrease in BMI of 0.12 kg/m(2) (95% CI 0.06, 0.18; P < 0.001). The mean BMI +/- SD of subjects in the highest quintile of free T4 concentration was 24.4 +/- 3.5 kg/m(2), compared with 26.1 +/- 3.8 kg/m(2) for the lowest quintile. The relationship between free T4 and BMI was statistically significant (adjusted for age and sex) in subjects who had never smoked (P = 0.001) and former smokers (P = 0.011), but not in current smokers (P = 0.77). There was no significant relationship between TSH and BMI: after adjustment for age and sex, each 1 mU/l increase in TSH was associated with an increase in BMI of 0.08 kg/m(2) (95% CI -0.16, 0.32; P = 0.53).


CONCLUSIONS
In euthyroid subjects, small differences in free T4 are associated with differences in BMI. This relationship is not present in current smokers. We speculate that this may be relevant to weight changes associated with smoking cessation.
Makepeace
Ashley E
AE
Department of Endocrinology and Diabetes, Sir Charles Gairdner Hospital, Nedlands, Western Australia, Australia.
Bremner
Alexandra P
AP
O'Leary
Peter
P
Leedman
Peter J
PJ
Feddema
Peter
P
Michelangeli
Valdo
V
Walsh
John P
JP
D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001315:Australia / Q000453:epidemiology; D015992:Body Mass Index; D003430:Cross-Sectional Studies; D015444:Exercise / Q000502:physiology; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D012907:Smoking / Q000097:blood* / Q000453:epidemiology* / Q000503:physiopathology; D013961:Thyroid Gland / Q000502:physiology* / Q000503:physiopathology; D013974:Thyroxine / Q000097:blood* / Q000737:chemistry; D055815:Young Adult
D013974:Thyroxine