Broome MR, Peterson ME.
In humans, thyroid nodules are common, occurring in over half of individuals over 50 years of age. Over 95% of these thyroid nodules are benign, small to moderate in size, and are not associated with clinical or biochemical signs of hyperthyroidism. Some of these patients that present with a single thyroid nodule are later found to have many benign enlarged nodules within the thyroid (ie, multinodular goiter), some of which will develop signs of hyperthyroidism (toxic nodular goiter; Plumber’s disease). Although the underlying cause of most human thyroid nodules is not known, a low-iodine diet, Hashimoto’s thyroiditis, or prior radiation treatment can contribute to nodule development. Historically, the most common medical treatment used in management of thyroid nodules in human patients is thyroid suppressive therapy, in which L-thyroxine is used to suppress pituitary TSH secretion and slow the growth of the thyroid nodule. In all human patients found to have a thyroid nodule, a major concern is the possibility of thyroid cancer, in which more aggressive therapy is required. The finding that thyroid replacement results in shrinkage of the nodule makes thyroid cancer less likely. Unfortunately, there is not clear evidence that treatment with L-thyroxine consistently shrinks thyroid nodules or even that shrinking small, benign nodules is necessary in most patients. Therefore, most authorities no longer recommend routine L T4 suppressive treatment for thyroid nodules except in certain conditions.