The role of enhanced thyroid-stimulating hormone (TSH) secretion, in the aetiology of thyroid cancer is not totally consistent. Circumstances and conditions which cause (e.g., iodine deficiency, through suboptimal intake in water and food) or indicate (e.g., goitre) increased TSH secretion have been associated to increased risk of thyroid cancer, most notably follicular and anaplastic carcinomas. Elevated incidence and mortality rates of thyroid cancer, however, are also found in areas were iodine intake is high (Hawaii, Iceland). At least in some countries (Switzerland), a favourable impact of the introduction of iodized salt on mortality from thyroid cancer has been reported. Elsewhere, the correction of iodine deficiency has coincided with elevations of diagnostic standards (e.g., spread of thyroid scintigraphy, ultrasound, and fine-needle biopsy) and corresponding increases in incidence of papillary carcinomas, often clinically silent, thus hampering a distinction of the two phenomena. Upward trends of papillary carcinoma incidence have, however, been seen in most affluent countries, irrespective of the iodine status of the population.