[Smoking and the thyroid]

The effects of smoking on the thyroid gland have been studied for years. The consequences of smoking on thyroid function and size are however still controversial. It is accepted that an increase in serum thyiocyanate, a potent inhibitor of iodine transport, may contribute to the development of thyroid dysfunction, particularly in geographical areas with borderline iodine deficiency. In the general population, smoking is associated with normal thyroid hormone levels, with a tendency to lower TSH levels and enlargement of thyroid size. There is an increased risk of developing over thyroid disease. The relationship between smoking and relapse of Graves' disease and the incidence and severity of Graves' ophthalmopathy are constantly reported. Smoking also has a potent nodular goitrigenic effect in low iodine areas. Despite the fact that smoking decrease both thyroid secretion and thyroid hormone action, hypothyroidism does not appear to be more frequent. Smoking does however increase the metabolic effects of hypothyroidism. Active and passive smoking have proven deleterious effects on the fetal thyroid. Paradoxically, there is evidence of reduced risk for thyroid cancer in smokers.