The effects of thyroxine-stimulated hypertrophy (TSH) were studied in the 
porcine left ventricular myocardium. Hypertrophy was produced in six adult pigs 
by administration of triiodothyronine (1 mg/kg; i.v.) for eight days. Six pigs 
served as controls. The degree of hypertrophy, determined by left 
ventricular-to-body weight ratio, was 47%. With hypertrophy there was a 
significant increase in heart rate, blood pressure and myocardial blood flows. 
Minimal coronary resistance measured during adenosine infusion was lower in the 
TSH group compared with the control group. Anatomic studies revealed a balanced 
proliferative response of mitochondria, myofibrils and the t-tubular system 
during TSH. Analysis of the microvasculature indicated that the capillary and 
arteriolar beds both experienced growth which paralleled myocyte growth during 
TSH. These results suggest that thyroxine administration promotes angiogenesis 
in the microvascular bed which provides a partial anatomic rationale for the 
lowered minimal coronary resistance.