Subclinical Hyperthyroidism And Atrial Fibrillation

Atrial fibrillation (AF) is a well-known manifestation of hyperthyroidism and occurs in 5 to 15 percent of patients with hyperthyroidism in different groups, and is especially prevalent over 60 years. Atrial fibrillation is an independent risk factor for arterial thromboembolism, stroke and congestive heart failure. Hence, the identification of risk factor for atrial fibrillation is important. Serum thyroid stimulating hormone (TSH) concentrations are a sensitive indicator of thyroid function. With the general availability of sensitive assays for measuring the concentration of TSH in serum, the entity of subclinical hyperthyroidism is being increasingly encountered. Subclinical hyperthyroidism is defined as a situation where the levels of the peripheral thyroid hormone are normal but serum TSH is low. It is not a rare finding; rates between 0.2% and 11.8% have been reported in different groups, according to age, sex, etc. Several studies found subclinical hyperthyroidism may be a risk factor for atrial fibrillation. We screened thyroid function markers in 234 patients in order to investigate the relationship between subclinical hyperthyroidism and atrial fibrillation.1. Objective:To investigate the relationship between subclinical hyperthyroidism and atrial fibrillation in Chinse.2. Materials and Methods2.1 PatientsNinety-fifth cases of patients with atrial fibrillation were divided into two groups (40-59 years and 60-79 years) according to age. Atrial fibrillation was diagnosed by electrocardiography performed at the biennial examinations. Some known risk factors for atrial fibrillation were excluded, including rheumatic heart disease, hypertension, coronary heart disease, congenital heart disease, dilated cardiomyopathy and sick sinus syndrome. Subjects who had taking iodine-containing preparations or thyroid hormone were also excluded.2.2 MethodsSerum levels of triiodothyronine ( T3 ) , thyroxine ( T4 ) , free triiodothyronine ( FT3 ) , free thyroxine ( FT4 ) and thyrotropin ( TSH ) were determined by immunochemiluminometric assay ( ICMA ) for ninety-fifth patients with atrial fibrillation and one hundred and thirty nine controls matched for age and sex.2.3 StatisticsAll statistical procedures were performed with SPSS 10.0 software package. Continuous variables were compared byt median of the Independent-Samples T Test. Differnces in distribution were compared by the Chi-square test or Fisher's exact test if appropriate. P values below 0.05 were considered statistically significant. 3.Results:In 40-59 years atrial fibrillation group showed significantly higher serum FT3 (4.68+1.26 p mol/L, P = 0.016) and higher serum FT4 (17.74+4.36 p mol/L , P = 0.025) than control group . No significant differences were found in serum TSH, T4and T3 levels among atrial fibrillation group and control group. In 60-89 years atrial fibrillation group showed significantly lower serum TSH (1.62+1.38 m IU/L, P = 0.006) than control group. No significant differences were found in serum T3, T4, FT3 and FT4 levels among atrial fibrillation group and control group. Seven hyperthyroidism patients (7.37%) and ten subclinical hyperthyroidism patients (10.5%) were detected among atrial fibrillation group. In 60-89 years the incidence of subclinical hyperthyroidism patients showed higher than in 10-59 years (x2= 3.918, P = 0.048). 4.Conclusions:This study showed significant correlation between atrial fibrillation and serum higher FT3, FT4 levels in 40-59 years. However, it showed significant correlation between atrial fibrillation and serum lower TSH in 60-79 years. Subclinical hyperthyroidism may be a risk factor for atrial fibrillation in old people.