Lipolytic and circulatory responses to noradrenaline infusion in hypothyroid subjects before and during thyroxine substitution.

The hypothyroid state in humans is associated with a diminished lipolytic response to noradrenaline in adipose tissue in vitro. In the present study we have investigated the situation in vivo in order to see if such a change in receptor response could be demonstrated in adipose tissue and in the circulatory system. The change in glycerol production rate, induced by an infusion of noradrenaline, was used as an index of adipose tissue adrenergic responsiveness. The results showed that the lipolytic response was decreased by about 50% in the hypothyroid state and that it was normalized when the substitution dose had been increased to 0.10-0.15 mg/day thyroxine. The circulatory response was monitored by measurements of blood pressure and pulse rate. The resting diastolic pressure was transiently lowered by substitution. Similarly the rise in systolic blood pressure induced by l-noradrenaline was transiently increased by substitution. Thus no clear-cut change in receptor response with substitution could be demonstrated by measuring BP and pulse rate only. This result could be due to the fact that the system is more complex than the adipose tissue. The finding of a reduced adrenergic receptor response in vivo in the adipose tissue is in accordance and extends earlier findings in vitro.