Isolated Unilateral Pleural Effusion Associated With Hypothyroidism: A Case Report and Review of the Literature.

A 77-year-old man with COPD and hypertension presented with progressive exertional dyspnea over 3 months. Physical examination revealed decreased breath sounds in the right lung base and signs suggestive of hypothyroidism, including dry skin and lateral eyebrow thinning. CT imaging showed emphysematous changes with moderate right pleural effusion. Pleural fluid analysis consistently showed borderline transudate/exudate characteristics. Comprehensive evaluation excluded heart failure, malignancy, pulmonary embolism and abdominal pathology. Subsequent thyroid function tests confirmed profound hypothyroidism (TSH: 49.6 μIU/mL). Levothyroxine therapy was initiated and titrated. Remarkably, follow-up imaging after 4 months demonstrated complete resolution of the pleural effusion, with normalised thyroid function and significant symptomatic improvement. Isolated pleural effusion represents an uncommon manifestation of hypothyroidism. The pathogenic mechanism may involve reduced albumin synthesis and increased vascular permeability, potentially mediated by TSH-stimulated VEGF production. This case is notable for the absence of classic myxedema symptoms while presenting significant pleural effusion. The diagnosis was established only after methodical exclusion of more common etiologies. Levothyroxine replacement therapy proved effective, resulting in complete effusion resolution.