One component of tobacco smoke is cyanide, which is converted to thiocyanate, which acts as an anti-thyroid agent, directly inhibiting iodide uptake and hormone synthesis. There are many other components of smoke that might have antithyroid action, decrease the binding of triiodothyronine (T3) to its receptors or its post-receptor actions in the liver, muscle, or other organs, or both.
Smoking and Graves' Disease / Thyroid Eye Disease
The most dramatic effect of smoking on the thyroid is its association with Graves' hyperthyroidism, and especially with Graves' ophthalmopathy. Whether smoking precedes Graves' hyperthyroidism (with or without ophthalmopathy) or not, there are more smokers than would be expected among those with these conditions.
According to a Jan. 27, 1993 article in the Journal of the American Medical Association smokers are twice as likely as nonsmokers to develop Graves' disease. According to that article, smoking also apparently worsens eye problems in people with Graves' disease.
Researchers also found that smoking reduces the effectiveness of treatments for thyroid eye disease. One study looked at the outcomes of 300 Graves' disease patients with mild eye symptoms who were treated with radioiodine alone or with steriods, and 150 with serious eye complications who received steroids and radiation therapy for their thyroid eye disease.
Among Graves' disease patients who had milder eye symptoms, smokers were more likely to progress to more serious thyroid eye disease than nonsmokers. Radioiodine and steroid treatment for thyroid eye disease was also four times more effective in dealing with the eye symptoms for nonsmokers than smokers. This same relationship also applied to patients with more serious thyroid eye disease. ("Smoking affects Graves' disease treatment," Annals of Internal Medicine, 1998;129:632-635.)
The Relationship Between Quitting Smoking and Thyroid Disease Onset
While I don't have an answer as to whether or not stopping smoking "triggers" problems with the thyroid -- as it anecdotally often appears to be -- it is clear that medical researchers have found that smoking can worsen hypothyroidism in people that already have it, and smoking can seriously affect thyroid function.
My theory is that smoking/nicotine creates an artificially high metabolism that masks the fatigue/lethargy commonly seen in hypothyroidism. When the smoker quits, this masking is removed, and the full effects of hypothyroidism on the metabolism and thyroid are felt.
And, for smokers with undiagnosed thyroid dysfunction, without proper thyroid hormone treatment, stopping seems to be a metabolic/weight gain double whammy, as they lose the appetite suppressant, metabolism-upping effects of nicotine, and experience the full effects of the hypothyroidism.
Why You Should Quit?
If you have thyroid disease, you should quit smoking now!! Smoking can and often will worsen your existing thyroid problem, and if you have Graves' disease or thyroid eye disease, you absolutely must quit smoking to help prevent further progression of problems or worsening of your eye problems. If you need support, About.com's Smoking Cessation site has many resources to help you successfully quit.
Update - 2013
There's a dietary supplement on the scene that is creating a great deal of interest among thyroid, autoimmune and inflammation patients. The supplement, called Anatabloc, is derived from anatabine - an alkaloid found in tobacco. Endocrinologist Paul Ladenson, MD from Johns Hopkins has been involved in researching anatabine and thyroid autoimmunity, and found that anatabine may have protective effects against autoimmune thyroid disease, and lower thyroid antibodies. In addition, early results from recent studies being conducted around the country are showing that Anatabloc may be able to reduce thyroid antibodies specifically, and modulate inflammation in general. If you're interested in learning more, here are some resources:
- An Immune System and Thyroid Upside for Tobacco? -- Post at the National Academy of Hypothyroidism, the non-profit group focused on educating patients and doctors about hypothyroidism Anatabloc website
- Caturegli P, et. al. "Anatabine ameliorates experimental autoimmune thyroiditis." Endocrinology. 2012 Sep;153(9):4580-7. Abstract at PubMed
- Star Scientific, Inc./Rock Creek Pharmaceuticals Report Positive Initial Results of ASAP Human Thyroid Health Study Showing Benefits in Immune System Support