The coincidence of thyroid disorders and breast cancer has long been a subject of debate. Associations with hyperthyroidism, hypothyroidism, thyroiditis and nontoxic goitre have been reported. Although no convincing evidence exists of a causal role for overt thyroid disease in breast cancer, the preponderance of published work favours an association with hypothyroidism. Geographical variations in the incidence of breast cancer have been attributed to differences in dietary iodine intake and an effect of iodide on the breast has been postulated. Recent reports have shown a direct association between thyroid enlargement, as assessed by ultrasound, and breast cancer. Although the exact mechanism for the demonstrated association between diseases of the thyroid and breast cancer remains to be elucidated, there is at least the possibility that the presence of thyroid abnormalities may influence breast cancer progression and this alone should stimulate awareness into the coincidence of the two disorders.