[The influence of thyroid function on the circadian rhythm of heart rate in elderly euthyroid subjects]

The influence of thyroid function on the circadian rhythm of heart rate was assessed using 24 hour electrocardiographic recordings in 196 elderly euthyroid subjects (78.4 +/- 8.1 years; male 60, 76.1 +/- 9.1 years; female 136, 79.4 +/- 7.4 years). Serum free T3, free T4 and TSH levels were measured by radioimmunoassay. To determine if heart rate fluctuates rhythmically with a circadian period, mean hourly heart rates in 24 hour electrocardiographic recordings were used to fit cosine curves by the statistical technique of least squares, and the parameters of the rhythm--the mesor, amplitude, acrophase, maximal and minimal hourly heart rates--were estimated. To investigate the factors influencing the diurnal variation of heart rate, we applied multiple regression analysis. The cosine curves were fitted with a P value of 0.01 or less in all subjects. The mesor, maximal and minimal hourly heart rates were inversely proportional to serum TSH concentrations in males. Neither amplitude nor acrophase had relation to the thyroid function in males. The mesor, maximal and minimal hourly heart rates were directly proportional to serum-free T4 concentrations and inversely proportional to serum TSH concentrations in females. The increase in the acrophase correlated with the increase in serum-free T4 levels, and the acrophase increased in proportion to serum-free T3 levels in females.