DISTURBED CIRCADIAN-RHYTHM OF BLOOD-PRESSURE AND HEART-RATE IN HYPERTHYROIDISM

Thyroid hormones increase the effects of catecholamines on metabolism and hyperthyroidism is associated with raised cardiac output and declined vascular resistance. These haemodynamic changes lead to an augmented systolic and a decreased diastolic blood pressure and a widened pulse pressure. Therefore it can be assumed, that the circadian variation of blood pressure and heart rate might be disturbed in hyperthyroidism. To evaluate the effects of hyperthyroidism on circadian rhythm of blood pressure and heart rate, in 10 untreated hyperthyroid patients (age 58.2 +/- 15.4 years, blood pressure 136.3 +/- 13.7/73.3 +/- 8.9 mmHg, heart rate 88.9 +/- 10.2 beats/min., TSH 0.07 +/- 0.04 myu/ml, T4 19.1 +/- 4.1 mug/dl, T3 3.3 +/- 1.2 ng/ml) and 10 euthyroid controls (age 51.3 +/- 12.8 years, blood pressure 129.3 +/- 10.1/80.7 +/- 7.9 mmHg, heart rate 74.0 +/- 9.3 beats/min.) a 24-hour monitoring of blood pressure and heart rate (SpaceLabs SL90207) was performed. The circadian variation of blood pressure and heart frequency was determined by the differences of averages during day time (8a.m-22p.m.) and during night time (22p.m.-8.a.m.). The day-night-differences of systolic and diastolic blood pressure were significantly reduced in the hyperthyroid group (difference systolic 6.5 +/- 4.9 mmHg, diastolic 7.9 +/- 7.2 mmHg) in comparison to the euthyroid group (systolic 15.8 +/- 11.1 mmHg, diastolic 9.3 +/- 6.4 mmHg), (p < 0.05). The day-night-difference of heart rate was also significantly impaired in the hyperthyroid group (8.2 +/- 4.9 mmHg beats/min. versus 15.8 +/- 11.1 beats/min.), (p < 0.05). in the hyperthyroid group, 39.8% of systolic blood pressure values were determined above 140 mmHg, 11.2% of diastolic values above 90 mmHg. The systolic blood pressure values in the euthyroid group were signicantly less frequent determined above 140 mmHg (23.4% of systolic values, p < 0.05), the diastolic values in this group were more frequent above 90 mmHg (15.2% of diastolic values). A correlation between the concentration of thyroid hormons and blood pressure values, heart rate and day-night-differences was not observed. The results show, that the variation of blood pressure and heart rate was significantly reduced in the hyperthyroid group in comparison to the euthyroid group (p < 0.05). It is concluded, that thyroid hormones influence the blood pressure and heart rate rhythm. The causes of impaired circadian blood pressure and heart rate variation could exist in a disturbed function of sympathetic nervous system due to an increased sensitivity to catecholamines induced by thyroid hormones.