To the Editor.
—We read with interest the article by Cobler et al1 in the September Archives. We recently studied masked thyroid dysfunction in 72 elderly patients with atrial fibrillation (40 men, 32 women; mean age, 75.6 years). Patients with mitral stenosis and apparent thyroid disease were excluded from our study. Thyroid dysfunction was diagnosed by determinations of triiodothyronine (T3) and thyroxine (T4) radioimmunoassay levels, T3 resin ratio, thyrotropin (thyroidstimulating hormone [TSH]) level, and TSH response to 250 μg of intravenous thyrotropin-releasing hormone (TRH). If hyperthyroidism was suspected, an iodine 123 uptake test or a thyrotropin-binding-inhibitory immunoglobulin assay or both were carried out. A TRH stimulation test was also performed in 30 age-matched controls without atrial fibrillation. None of the controls had an abnormal TRH-stimulated TSH response. In patients with atrial fibrillation, hyperthyroidism was found in five cases (6.9%), while hypothyroidism was found in six cases