Some unexpected effects of thyroid hormone on blood vessels could explain why people with thyroid disorders often feel too hot or too cold.

Jens Mittag of the Karolinska Institute in Stockholm and his colleagues found that mice with a mutation in a thyroid-hormone receptor burn fat at a higher rate than normal, but show no increase in their body temperatures.

Thermal imaging revealed that the mutant mice lost more heat than usual through their tails because the tail arteries were not constricting properly. This, in turn, boosted the rate at which the mice burned brown fat — an effect that was reversed when the mice were given a drug that stimulates blood-vessel constriction.

Credit: AMY WARNER

Proc. Natl Acad. Sci. USA http://doi.org/nv7 (2013)