Serum TSH is positively associated with BMI.

Area of Science:

• Endocrinology
• Metabolic Health
• Public Health

Background:

• Thyroid-stimulating hormone (TSH) plays a crucial role in regulating metabolism.
• The relationship between TSH levels within the normal range and body mass index (BMI) remains incompletely understood.
• Obesity is a significant public health concern with complex etiological factors.

Purpose of the Study:

• To investigate the association between serum TSH levels within the normal reference range and BMI.
• To explore potential differences in this association based on smoking status.
• To examine the longitudinal relationship between changes in TSH and BMI.

Main Methods:

• Cross-sectional analysis of 6164 participants from the fifth Tromsø study (2001).
• Longitudinal analysis of 1867 participants from the Tromsø study (1994/1995 and 2001).
• Measurements included serum TSH, height, weight, and smoking status, with analyses stratified by smoking status.

Main Results:

• A significant positive association was found between serum TSH and BMI in nonsmokers within the normal TSH range.
• Nonsmoking women in the highest TSH quartile had a 1.4 kg/m² higher mean BMI compared to the lowest quartile.
• In nonsmokers, higher delta TSH was associated with a greater increase in BMI over time in the longitudinal cohort.

Conclusions:

• Serum TSH within the normal range is positively associated with BMI in nonsmokers.
• This association suggests a potential role for subtle variations in thyroid function within the euthyroid state in weight regulation.
• No significant association was observed between TSH and BMI in smokers.