Informing & Supporting Thyroid Patients Since 1985
Informing & Supporting Thyroid Patients Since 1985
Thyroid Research 
Thyroid Research Archive Thyroid Hormone Action
(November 2006)
The background of the study. Thyroid hormones, primarily triiodothyronine (T3), act by binding to specific receptors, and the T3-receptor complexes activate or inhibit the activity of many genes. However, some actions of the hormone are very rapid, and cannot be explained by this mechanism. This study tested the hypothesis that T3 rapidly stimulates an enzyme in blood vessels known as nitric oxide synthase, leading to an increase in nitric acid, a substance that dilates blood vessels, resulting in an acute decrease in blood pressure and an increase in cerebral blood flow.
How the study was done and the results of the study. T3 receptors were detected in endothelial cells (cells that line blood vessels), indicating that T3 might act on these cells. Incubation of these cells with T3 resulted in dose-dependent increases in the activity of endothelial-cell nitric oxide synthase and nitric oxide synthesis in 10 to 20 minutes.
Administration of T3 to normal mice resulted in a decrease in mean blood pressure in 5 minutes, and by 30 minutes it had fallen from 84 to 80 mm Hg. The fall was greater in hypothyroid mice. In contrast, there was little or no fall in mean blood pressure in normal or hypothyroid mice lacking endothelial nitric oxide synthase.
In normal mice, administration of T3 increased cerebral blood flow, whereas there was no increase in mice lacking endothelial nitric acid synthase. In mice in which one cerebral artery was occluded for 2 hr and then reopened, administration of T3 before occlusion caused less neurologic damage and decreased the size of the brain injury.
The conclusions of the study. T3 rapidly increases nitric acid synthase activity and nitric oxide production in blood vessels, which causes dilatation of the vessels, increased cerebral blood flow, and decreased brain injury after cerebral artery occlusion in mice.
The original article. Hiroi Y, Kim HH, Ying H, Furuya F, Huang Z, Simoncini T, Noma K, Ueki K, Nguyen NH, Scanlan TS, Moskowitz MA, Cheng SY, Liao JK. Rapid nongenomic actions of thyroid hormone. Proc Natl Acad Sci USA 2006;103:14104-9
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