The normal thyroid gland is 2cm wide, 2cm deep, and 4-5cm tall. The thyroid may enlarge for a variety of reasons, including iodine deficiency, thyrotoxicosis, nodule formation or cancer. Goitre is simply the medical word for thyroid enlargement. Goitre is endemic in regions of the world of significant iodine deficiency eg. mountains of China. The average Australian diet is only mildly iodine deficient and this has been improved recently with the mandatory fortification of bread with iodised salt since 2009.
Thyroid nodules are very common, particularly in women and in people over the age of 60. The widespread use of ultrasound and other imaging techniques has dramatically increased the number of people with identified thyroid nodules (most of these nodules would otherwise have been undetected). The challenge is determining which nodules can be ignored and which need further investigation. Advances in ultrasound technology have enabled the identification of particular nodule appearances that are associated with thyroid cancer, vs those associated with benign disease. In general, thyroid nodules >1cm in size should have a fine needle biopsy, but this is dependent on ultrasound features and also on thyroid function.
Some nodules may become overactive and cause hyperthyroidism. These nodules can be easily treated with a a tablet of radioactive-iodine which destroys the overactive nodule and usually causes significant shrinkage of the nodule.
Some nodules, even when benign (not cancerous), may grow to a size where they need treatment. This may simply be for cosmetic reasons or because the nodule is pressing on surrounding structures causing discomfort (coughing, difficulty swallowing, hoarse voice). Treatment options here include needle aspiration (if there is a significant fluid component), ablation with radio-iodine or surgical removal. The best treatment depends on your individual circumstances.