Context. Thyroid hormone regulates several cardiovascular functions and low triiodothyronine (T3) levels are frequently associated with cardiovascular diseases. Whether T3 exerts any acute and direct effect on endothelial function in humans is unknown.

Objective. To clarify whether acute changes in serum T3 concentration affect endothelial function.

Design. Double blind, placebo controlled

Setting. University hospital

Patients or Other participants. Ten healthy subjects (age: 24 ± 1 yr).

Interventions. T3 (or placebo) was infused for 7 h into the brachial artery to raise local T3 to levels observed in moderate hyperthyroidism. Vascular reactivity was tested by intra-arterial infusion of vasoactive agents.

Main Outcome Measures. Changes in forearm blood flow (FBF) measured by plethysmography.

Results. FBF response to the endothelium-dependent vasodilator acetylcholine was enhanced by T3 (P = 0.002 for the interaction between T3 and acetylcholine). The slopes of the dose-response curves were 0.41 ± 0.06 ml/dl · min/µg and 0.23 ± 0.04 in the T3 and placebo study, respectively; P = 0.03). T3 infusion had no effect on the FBF response to sodium nitroprusside. T3 potentiated the vasoconstrictory response to norepinephrine (P = 0.006 for the interaction). Also the slopes of the dose-response curves were affected by T3 (1.95 ± 0.77 ml/dl · min/mg and 3.83 ± 0.35 in the placebo and T3 study, respectively; P < 0.05). The increase in basal FBF induced by T3 was inhibited by L-NMMA.

Conclusions. T3 exerts direct and acute effects on the resistance vessels by enhancing endothelial function and norepinephrine-induced vasoconstriction. The data may help clarify the vascular impact of the low T3 syndrome and open to potential therapeutic strategies.