Out of 20 subjects selected for refractory arrhythmias, amiodarone therapy (200 mg/day) was efficacious in 85%. No statistically significant variations in electrocardiographic parameters (QTc) were observed; similarly, there was little evidence of side effects 1 year after initiation of treatment. These results were most likely due to the low daily dosage administered. We observed: 1) a significant increase in rT3 levels; 2) a decrease in TT3; 3) a uniform homeostasis of free fraction (FT3;FT4) These effects are all characteristic patterns of a "Low T3 Syndrome". The dosage of circulating amiodarone in 6 patients with borderline hormonal status (3 hyper- and 3 hypothyroidism) was not found to be an efficacious test for therapeutic monitoring. Identification of a statistically significant linear regression relationship between cumulative dose of amiodarone and rT3 levels may be a useful test in clinical practise for establishing more appropriate therapeutic dosages. Furthermore, it provides a guideline for threshold levels (maximum rT3 = 100-110 ng/dl) which are in close association with several side effects.