title: Exogenous subclinical hyperthyroidism: effect on the cardiovascular system.
abstract: OBJECTIVE
  To evaluate the effects of exogenous subclinical hyperthyroidism on left ventricular structure and function.
  
  
  METHODS
  Twenty-three patients of both sexes, aged 27 to 70 years, with a diagnosis of exogenous subclinical hyperthyroidism (serum thyroid-stimulating hormone [TSH] ≤ 0.4mU/ml and normal free thyroxine [FT4]) were evaluated. The patients had been taking levothyroxine in suppressive doses for an average of 6.7 years (1 to 35 years). Twenty euthyroid individuals matched for age, sex and body mass index were selected as controls. A medical history was obtained and symptoms of thyrotoxicosis were quantified in all subjects. To evaluate left ventricular structure and function, as well as atrial conduction time, a two-dimensional echocardiogram and pulsed echo and tissue echo Doppler with electrocardiography monitoring were performed. The index of hemodynamic compensation of the left ventricular mass was calculated.
  
  
  RESULTS
  Hyperthyroid symptom scores were significantly higher in patients than in controls (p=0.0001). A positive correlation was found between hyperthyroidism scores and FT4 (p=0.005) and ejection fraction (p=0.04) and a negative correlation was found with TSH levels (p=0.03). End-diastolic volume, stroke volume, cardiac output and stroke work were significantly higher in patients with SH (p=0.04, p=0.02, p=0.05 and p=0.01, respectively). A positive correlation was found between fractional shortening and FT4 level (p=0.022) and levothyroxine dose (p=0.016) and between stroke work and FT4 level (p=0.034). Left ventricular mass, diastolic function and atrial conduction time were similar in patients and controls.
  
  
  CONCLUSIONS
  Our study suggests that patients with exogenous subclinical hyperthyroidism have hemodynamic changes resulting from adaptation to the biological effects of levothyroxine on the cardiovascular system. However, structural changes are not produced.
authors.affiliation: Unidad de Endocrinología. Instituto Autónomo Hospital Universitario de Los Andes. Universidad de Los Andes. Mérida. Venezuela.
authors.forename: Karol
authors.initials: K
authors.lastname: Federico Moreno
authors.forename: Mariela
authors.initials: M
authors.lastname: Paoli de Valeri
authors.forename: Rodolfo
authors.initials: R
authors.lastname: Odreman
authors.forename: Tulio
authors.initials: T
authors.lastname: Núñez
authors.forename: Gabriela
authors.initials: G
authors.lastname: Arata-Bellabarba