In the search for a clinical correlate to thyroid function, correlations were sought between the plasma level of thyroid hormones and the cardiac function, assessed by heart rate and the systolic time intervals. The present study is the first to 1) use a curvilinear correlation and 2) do so in the entire hormone range. A third degree Chebychew's regression equation was used to enable curvilinear correlation. In a population of 10 hypothyroid, 14 euthyroid, and 25 hyperthyroid patients a statistically significant curvilinear correlation was found between the thyroid hormone levels and the systolic time intervals. The correlation coefficients showed the heart rate equally correlated to the thyroid hormone levels as the best correlated of the systolic time intervals. The correlation between the heart rate and T3 was almost linear, with a heart rate increase of 4 beats/minute with each increase in plasma T3 of 1 nmol/l.
Conclusion: The thyroid function reflects in the cardiac function in a curvilinear regression between T3 and systolic time intervals and in an almost linear regression between T3 and heart rate. The variation, however, is too great and the slope too low, to suggest the use of the heart rate as a clinical correlate to thyroid function.