Tobacco smoke may have as many as 4000 different chemicals, depending on the type of tobacco and how it is smoked. At least 60 of these chemicals are toxic. Smoking can affect thyroid function. One component of tobacco smoke, thiocyanate, (which is derived from hydrogen cyanide), increases iodine excretion and inhibits iodine uptake by the thyroid. Thiocyanate also inhibits thyroid hormone synthesis. Hydroxypyridine metabolites and benzpyrenes may also interfere with thyroid function.

Some studies show smoking to smoking to increase thyroid function, while others show it to suppress thyroid. The authors suggest that this variation may be related to iodine intake—low intake giving a tendency for smoking to lower thyroid function. Smoking seems to have negative effect on the thyroid gland seems when thyroid function is slightly compromised. Smoking may contribute to the high incidence of subclinical hypothyroidism – 10% in some studies.

In patients with hypothyroidism, smoking has no effect on blood levels of thyroid hormone. However, smoking may possibly aggravate both the symptoms and biochemical effects of hypothyroidism. There is also evidence that smoking increases the risk for contracting Graves’ disease. It may also sensitize retro-orbital tissue to whatever factor causes ophthalmopathy