A 69-year-old woman was admitted for the treatment of marked pleural effusions and peripheral edema. Analytical studies of the pleural effusion revealed exudates. Culture for bacterial organisms and tuberculosis were negative, and cytology was normal. She had a mediastinal tumor at the age of 61 and regular follow-up showed no evidence of malignancy. She underwent the mediastinal tumor resection, because we thought this was the cause of her symptoms. However, her clinical symptoms persisted after surgery. Next, we noticed subclinical hypothyroidism, in which serum TSH level was elevated with concomitant normal thyroid hormone levels. In addition, serum vascular endothelial growth factor (VEGF) levels, which have been reported to be related to the pathophysiology of the extravascular volume overload, were elevated. Although her TSH level was slightly elevated (15.4 microU/ml), we started thyroid hormone replacement therapy. This therapy gradually ameliorated her clinical manifestation and abnormal laboratory data, including elevated VEGF levels. These observations indicate that even subclinical hypothyroidism may cause severe clinical manifestations. Furthermore, elevated VEGF may be a contributing factor in the pathogenesis of extravascular volume overload in hypothyroid patients.