Thyroidal vascular responsiveness to parasympathetic stimulation is increased in hyperthyroidism.

It has been suggested that thyroid blood flow (TBF) is regulated by both parasympathetic and sympathetic nerves. Because thyroxine (T4) pretreatment increases the sensitivity of the thyroid to the effects of thyrotropin, the present study was conducted to determine whether T4 pretreatment can also sensitize the thyroid to the effect of parasympathetic stimulation on TBF. Untreated or T4-pretreated rats were anesthetized, and both superior laryngeal nerves (SLN) were transected. TBF was continuously monitored by laser Doppler flowmetry (LDF), and thyroid vascular conductance (TVC) was also determined by the microsphere technique. Stimulation of the SLN had no effect on TBF or TVC in untreated rats when measured by LDF or microspheres. In contrast, stimulation of the SLN after T4 pretreatment increased TBF by 65 +/- 21% over prestimulus levels as measured by LDF. TVC was also increased significantly (P < 0.05) in these rats compared with TVC in a nonstimulated T4-pretreated group. To examine the role of muscarinic receptor activation in the mediation of these increases in TVC, T4 pretreated rats were given saline or atropine prior to SLN transection. Stimulation of the SLN in T4-pretreated rats given saline increased TVC significantly (P < 0.05) compared with TVC in the nonstimulated saline-treated or atropine-treated group. In contrast, TVC in the stimulated group given saline was not significantly different from the group that was stimulated after atropine injection. Our results suggest that the thyroidal vascular responsiveness to parasympathetic stimulation is increased in the hyperthyroid condition.