INTRODUCTION

Subclinical hyperthyroidism is an increasingly recognized entity that is defined as normal serum free thyroxine (T4) and tri-iodothyronine (T3) levels with thyroid-stimulating hormone (TSH) level suppressed below the normal range and usually undetectable. Like overt hyperthyroidism, endogenous subclinical hyperthyroidism is due to Graves disease, multinodular goitre or an autonomously functioning thyroid nodule (1). The prevalence of subclinical hyperthyroidism ranges from 0.6% to 16% [13, 14], depending on the definition used (that is, TSH concentration lower than normal, -0.1 mU/L or undetectable in a given assay), the sensitivity of the method used to measure TSH concentrations and iodine intake in the study sample (2). There are some cases in the literature with bradyarrhythmic conduction disturbance accompanying thyrotoxicosis (3, 4) but to our best knowledge this is the first case with subclinical hyperthyroidism presenting with recurrent syncope due to sick sinus syndrome in the literature.