Multiple thyroid cysts: a benign and stable clinical entity based on longitudinal data.

Multiple thyroid cysts are commonly detected by ultrasonography, and are considered clinically insignificant. On the other hand, polycystic thyroid disease (PCTD) has been proposed as a cause of antibody-negative hypothyroidism in iodine-replete regions; however, the clinical characteristics and natural history of multiple thyroid cysts as a broader entity remain unclear. To elucidate these issues, we retrospectively reviewed 7,727 consecutive new outpatients at a high-volume thyroid center between 2013 and 2016. Multiple thyroid cysts were defined as ≥4 cysts (≥3 mm in diameter) in the absence of thyroid autoantibodies. Among all patients, 164 (2.1%) met the criteria. The mean age was 58.3 years, and 91.0% were female. At presentation, 86.6% were euthyroid, 6.1% and 6.7% had subclinical hypothyroidism and subclinical hyperthyroidism, respectively, and one patient had Graves' disease. Thus, approximately 6% of multiple thyroid cysts may be PCTD. Forty-six patients were followed up for ≥5 years (mean 9.8 years) without iodine restriction. Thyroid function remained stable in most cases, with no significant progression to overt dysfunction and no requirement for surgical or interventional treatment. Ultrasonographic findings ranged from numerous diffuse cysts throughout the gland to several scattered or localized cysts. Some cysts remained stable, some increased, and others decreased in size in the follow-up periods. The cyst distribution patterns (diffuse versus localized) were not associated with thyroid function. Therefore, multiple thyroid cysts may generally represent benign and stable clinical entities, supporting conservative management with periodic monitoring in most patients.