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Tobacco Smoking and Thyroid Function: Is Weight Gain a Confounder?—Reply

Tobacco Smoking and Thyroid Function: Is Weight Gain a Confounder?—Reply In reply In assessing whether effects of smoking on thyroid function may be confounded by body mass, it is important to consider the possible causal directions of the association between thyroid function and body mass. A Danish population-based study showed that body mass index (BMI) was positively associated with thyrotropin level but negatively associated with free thyroxine level, suggesting that thyroid function may influence BMI.1 With this causal direction, BMI does not fulfill the criteria for being a confounder or a mediator of the associations of smoking with thyroid function.2,3 However, other studies indicate that thyroid function may be influenced by weight reduction among obese individuals4 or by weight gain in patients with anorexia nervosa.5 We had no information about weight change in our study and cannot address whether change in weight may influence thyroid function. However, adjustment for current BMI did not substantially influence the associations between smoking status and thyroid function in our data. Thus, our results do not indicate that the associations of smoking with thyroid function are confounded or mediated by body mass. Correspondence: Dr Åsvold, Department of Public Health, Faculty of Medicine, Norwegian University of Science and Technology, N-7489 Trondheim, Norway (bjorn.o.asvold@ntnu.no). References 1. Knudsen NLaurberg PRasmussen LB et al. Small differences in thyroid function may be important for body mass index and the occurrence of obesity in the population. J Clin Endocrinol Metab 2005;90 (7) 4019- 4024PubMedGoogle ScholarCrossref 2. Hernán MAHernandez-Diaz SWerler MMMitchell AA Causal knowledge as a prerequisite for confounding evaluation: an application to birth defects epidemiology. Am J Epidemiol 2002;155 (2) 176- 184PubMedGoogle ScholarCrossref 3. Rothman KJGreenland S Precision and validity in epidemiologic studies. Rothman KJGreenland S Modern Epidemiology. 2nd ed. Philadelphia, PA Lippincott Williams & Wilkins1998;Google Scholar 4. Kok PRoelfsema FLangendonk JG et al. High circulating thyrotropin levels in obese women are reduced after body weight loss induced by caloric restriction. J Clin Endocrinol Metab 2005;90 (8) 4659- 4663PubMedGoogle ScholarCrossref 5. Onur SHaas VBosy-Westphal A et al. L-tri-iodothyronine is a major determinant of resting energy expenditure in underweight patients with anorexia nervosa and during weight gain. Eur J Endocrinol 2005;152 (2) 179- 184PubMedGoogle ScholarCrossref http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Archives of Internal Medicine American Medical Association

Tobacco Smoking and Thyroid Function: Is Weight Gain a Confounder?—Reply

Tobacco Smoking and Thyroid Function: Is Weight Gain a Confounder?—Reply

Abstract

In reply In assessing whether effects of smoking on thyroid function may be confounded by body mass, it is important to consider the possible causal directions of the association between thyroid function and body mass. A Danish population-based study showed that body mass index (BMI) was positively associated with thyrotropin level but negatively associated with free thyroxine level, suggesting that thyroid function may influence BMI.1 With this causal direction, BMI does not fulfill the...
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Publisher
American Medical Association
Copyright
Copyright © 2008 American Medical Association. All Rights Reserved.
ISSN
0003-9926
eISSN
1538-3679
DOI
10.1001/archinternmed.2007.20
Publisher site
See Article on Publisher Site

Abstract

In reply In assessing whether effects of smoking on thyroid function may be confounded by body mass, it is important to consider the possible causal directions of the association between thyroid function and body mass. A Danish population-based study showed that body mass index (BMI) was positively associated with thyrotropin level but negatively associated with free thyroxine level, suggesting that thyroid function may influence BMI.1 With this causal direction, BMI does not fulfill the criteria for being a confounder or a mediator of the associations of smoking with thyroid function.2,3 However, other studies indicate that thyroid function may be influenced by weight reduction among obese individuals4 or by weight gain in patients with anorexia nervosa.5 We had no information about weight change in our study and cannot address whether change in weight may influence thyroid function. However, adjustment for current BMI did not substantially influence the associations between smoking status and thyroid function in our data. Thus, our results do not indicate that the associations of smoking with thyroid function are confounded or mediated by body mass. Correspondence: Dr Åsvold, Department of Public Health, Faculty of Medicine, Norwegian University of Science and Technology, N-7489 Trondheim, Norway (bjorn.o.asvold@ntnu.no). References 1. Knudsen NLaurberg PRasmussen LB et al. Small differences in thyroid function may be important for body mass index and the occurrence of obesity in the population. J Clin Endocrinol Metab 2005;90 (7) 4019- 4024PubMedGoogle ScholarCrossref 2. Hernán MAHernandez-Diaz SWerler MMMitchell AA Causal knowledge as a prerequisite for confounding evaluation: an application to birth defects epidemiology. Am J Epidemiol 2002;155 (2) 176- 184PubMedGoogle ScholarCrossref 3. Rothman KJGreenland S Precision and validity in epidemiologic studies. Rothman KJGreenland S Modern Epidemiology. 2nd ed. Philadelphia, PA Lippincott Williams & Wilkins1998;Google Scholar 4. Kok PRoelfsema FLangendonk JG et al. High circulating thyrotropin levels in obese women are reduced after body weight loss induced by caloric restriction. J Clin Endocrinol Metab 2005;90 (8) 4659- 4663PubMedGoogle ScholarCrossref 5. Onur SHaas VBosy-Westphal A et al. L-tri-iodothyronine is a major determinant of resting energy expenditure in underweight patients with anorexia nervosa and during weight gain. Eur J Endocrinol 2005;152 (2) 179- 184PubMedGoogle ScholarCrossref

Journal

Archives of Internal MedicineAmerican Medical Association

Published: Jan 14, 2008

Keywords: thyroid function tests,weight gain

References