Is the Resistance of Large Conduit Arteries Also Decreased in Thyrotoxic Patients with Graves' Disease?

Longitudinal arterial resistance (RL) originates during blood flow in large conduit arteries while peripheral resistance(RP) reflects the tension of arterioles. Using a new noninvasive ultrasonic system of assessment of mechanical properties of arterial walls, we aimed to discover whether both RL and RP are affected by excess of thyroid hormones (TH). We compared two groups: hyperthyroid (Graves' group, n = 25) and euthyroid (control group, n = 23). RL and RP values were obtained from the analysis of the four-element lumped parameters model describing the vascular input impedance. The vascular input impedance was determined on the basis of simultaneous measurements of blood pressure, blood flow and arterial diameter using oscillometric and ultrasonic methods. RL and RP were lower in the Graves' group (3.71 ± 1.98 versus 4.49 ± 1.81 10⁸kgm^–4s^–1, p < 0.05 and 8.43 ± 3.56 versus 11.23 ± 2.69 0⁸kgm^–4s^–1, p < 0.00002, respectively). There was strong correlation between RP and TH in the Graves' group (r_s= –0.6710, p = 0.000001 for RP and free triiodothyronine (FT3); r_s= –0.4683,p < 0.001 for RP and free thyroxine [FT4]). This relationship remained significant after the statistical correction for total cholesterol (r_p= –0.7064 p = 0.0000001 for RP and FT3). No similar correlation between RL and TH was found what suggests a different character of relations between TH and RL and between TH and RP.