Cigarette smoking and thyroid hormone levels in males

Area of Science:

• Endocrinology
• Environmental Health

Background:

• Cigarette smoking is associated with thyroid disease, but findings on its effects on thyroid hormone levels are inconsistent.
• Some studies report elevated thyroid hormones in smokers, while others indicate decreased levels.

Purpose of the Study:

• To investigate the relationship between cigarette smoking and thyroid hormone levels in a cohort of Vietnam-era male US Army veterans.
• To explore potential mechanisms underlying the observed effects of smoking on thyroid function.

Main Methods:

• Secondary analysis of data from 4462 Vietnam-era male veterans (aged 31-49) from the Vietnam Experience Study (1985-1986).
• Comparison of thyroid hormone levels (thyroxine, thyroid stimulating hormone) between current smokers (n=1962) and non-smokers (n=2406).
• Control for thyroxine-binding globulin and testosterone levels, and analysis by smoking intensity (heavy vs. light smokers).

Main Results:

• Current smokers exhibited higher thyroxine and lower thyroid stimulating hormone levels compared to never and former smokers.
• The difference in thyroxine levels between smokers and non-smokers decreased after adjusting for thyroxine-binding globulin and testosterone.
• Heavy smokers showed a less pronounced increase in thyroxine levels compared to light smokers when contrasted with non-smokers.

Conclusions:

• Tobacco smoke influences thyroid function through at least two distinct pathways.
• One mechanism involves elevated thyroxine-binding globulin and testosterone in smokers.
• Another mechanism relates to higher concentrations of thyrotoxins in tobacco smoke, particularly in heavy smokers.