Smoking cessation is followed by a sharp but transient rise in the incidence of overt autoimmune hypothyroidism - a population-based, case-control study.
BACKGROUND
Current smoking is associated with a low prevalence of thyroid autoantibodies. On the other hand, smoking withdrawal enhances thyroid autoantibody level and may be a risk factor for the development of hypothyroidism. The aim of this study was to assess the association between smoking habits (smoking cessation in particular) and development of autoimmune hypothyroidism.


METHODS
Population-based, case-control study.


METHODS
Cases (n = 140) newly diagnosed with primary autoimmune overt hypothyroidism were identified prospectively by population monitoring (2,027,208 person-years of observation) of all thyroid function tests performed in the two well-defined geographical areas. Individually, age-, sex- and region-matched euthyroid controls (n = 560) were simultaneously included from the same population.


METHODS
Participants gave details on smoking habits including smoking withdrawal and other lifestyle factors. Smoking habits were verified by measuring urinary cotinine (a nicotine metabolite).


RESULTS
Incident hypothyroidism was very common in people who had recently stopped smoking: OR vs never smokers (95%-CI); quit smoking <1 years, 7·36 (2·27-23·9); 1-2 years, 6·34 (2·59-15·3); 3-10 years, 0·75 (0·30-1·87); >10 years, 0·76 (0·38-1·51). Results were consistent in both sexes and irrespective of age. Within two years after smoking cessation, the percentage of hypothyroid cases attributable to cessation of smoking was 85%. The current smoking was not associated with altered risk of developing overt hypothyroidism [OR, 0·92 (0·57-1·48)].


CONCLUSIONS
The risk of having overt autoimmune hypothyroidism diagnosed is more than 6-fold increased the first 2 years after cessation of smoking. Clearly, smoking cessation is vital to prevent death and severe disease. However, awareness of hypothyroidism should be high in people who have recently quit smoking, and virtually any complaint should lead to thyroid function testing.
Department of Endocrinology & Medicine, Aalborg Hospital, Aarhus University Hospital, Aalborg, Denmark. carle@dadlnet.dk
Allan
A
Carlé
Inge
I
Bülow Pedersen
Nils
N
Knudsen
Hans
H
Perrild
Lars
L
Ovesen
Lone
L
Banke Rasmussen
Torben
T
Jørgensen
Peter
P
Laurberg
D000293:Adolescent; D000328:Adult; D000368:Aged; D016022:Case-Control Studies; D005260:Female; D006801:Humans; D007037:Hypothyroidism / Q000453:epidemiology* / Q000209:etiology*; D008297:Male; D008875:Middle Aged; D011446:Prospective Studies; D016540:Smoking Cessation; D011795:Surveys and Questionnaires; D013967:Thyroiditis, Autoimmune / Q000453:epidemiology / Q000209:etiology; D055815:Young Adult