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Smokers have less thyroid autoimmunity and lower serum thyrotropin concentrations than nonsmokers

(March 2005)

The background of the study. Cigarette smoking has both inhibitory and stimulatory effects on thyroid function, and it is a risk factor for Graves’ hyperthyroidism and especially ophthalmopathy. This study assessed the relationships between smoking and serum thyrotropin (TSH) and antithyroid antibody concentrations in a large group of people in the United States.

How the study was done. The study subjects were 16,046 people who had participated in the third National Health and Nutrition Survey in 1988–1994, and for whom demographic, smoke exposure, and serum TSH, antithyroid peroxidase (anti-TPO) antibody, and antithyroglobulin (anti-Tg) antibody results were available. Smoking or smoke exposure was determined by measuring serum cotinine, a metabolite of nicotine; people with concentrations >15 ng/ml were designated as active smokers, and those with concentrations ≤15 ng/ml as nonsmokers.

The results of the study. There were 5134 active smokers (32 percent) and 10,912 nonsmokers (68 percent). More men than women were active smokers. Fewer active smokers than nonsmokers had high (>4.5 mU/L) serum TSH concentrations (2.6 vs. 5.4 percent). Serum TSH concentrations were low (<0.1 mU/L) in 0.6 percent of active smokers and 0.3 percent of nonsmokers, and the concentrations were slightly low (0.1 to 0.4 mU/L) in 2.2 and 1.2 percent, respectively. Fewer smokers than nonsmokers had high serum antithyroid antibody concentrations (11 vs. 18 percent). Among subjects with high serum antithyroid antibody concentrations, the likelihood of a high serum TSH concentration was lower in active smokers than in nonsmokers.

The conclusions of the study. Among smokers, serum TSH concentrations are low more often and serum TSH and antithyroid antibody concentrations are high less often than in nonsmokers.

The original article. Belin RM, Astor BC, Powe NR, Ladenson PW. Smoke exposure is associated with a lower prevalence of serum thyroid autoantibodies and thyrotropin concentration elevation and a higher prevalence of mild thyrotropin concentration suppression in the third National Health and Nutrition Examination Survey (NHANES III). J Clin Endocrinol Metab 2004;89:6077-86.

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