Adverse effects of subclinical hyperthyroidism

Evidence of the harmful effects of subclinical hyperthyroidism and serum thyrotropin concentration below the lower limit of the laboratory has come only from observational and epidemiological studies. The cumulative incidence of atrial fibrillation at 10 years among people 60 years or older is 28% (relative risk [RR] 3.1) for thyrotropin concentrations less than 0.1, and 16% (RR 1-6) for thyrotropin concentrations of 0.1 to 0.4, and 11% for normal thyrotropin values.8 Some studies also show increased heart rate, premature atrial contractions, increased left-ventricular mass index, and decreased left-ventricular filling with subclinical hyperthyroidism.5 The Rotterdam prospective study10 showed that, among a random sample of 1843 participants, a serum thyrotropin concentration of less than 0.4 mU/L, without increased serum thyroxine concentrations, was associated with a three-fold increased risk of dementia and Alzheimer's disease (RR 3.5). In a recent report, the risk of hip fractures was increased three-fold among women with thyrotropin concentrations of less than 0.1 mU/L compared with women with normal thyrotropin concentrations.11 The risk of hip fractures was not increased with thyrotropin concentrations of OM to 0.5 mU/L. However, the same study showed a 2.8-fold increase in the risk of vertebral fracture with a thyrotropin concentration of 0.1-0.5 mU/L and a 4.4-fold increase with thyrotropin concentrations of less than 0.1 mU/L. Although free-thyroxine values were not reported, the findings are suggestive of adverse effects of subclinical hyperthyroidism on the hip and possible adverse effects of borderline to low thyrotropin concentrations on the spine.