The role of thyroid hormone (TH) in adult mammalian brain function including cholinergic neurotransmission is illusive, and its clinical application in recovery of mood disorder during altered thyroidal conditions appears to be dispersed. In this study, we explored whether TH supplementation helps in recovery of adult brain status during thyroid dysfunction. We observed a dose-dependent stimulation of acetylcholinesterase (AChE) activities by triiodothyronine (T3) in the synaptosomes prepared from adult rat cerebral cortex. The AChE activation in both propylthiouracil (PTU)-treated and T3-treated animals remained elevated indicating the presence of an autoregulatory mechanism for thyroid hormone levels in altered thyroidal conditions. Additive effects were also found with gradual decreases in the enzyme activities at 24, 48 and 72 h after single-injection of T3 to the PTU-treated rats. It appears that T3 exhibit a stimulatory effect on cholinergic neurotransmission in adult mammalian brain. Moreover, the histomorphological observation of thyroid gland revealed the disappearance of colloids and changes of follicular structures in 14-day PTU-treated condition. But the colloids reappeared and follicular structures normalized after a single injection of T3 to PTU-treated animals gradually at 24, 48 and 72 h. The results therefore, indicates that T3 supplementation can augment hypothyroid induced alterations of central cholinergic neurotransmission vis-a-vis peripheral thyroid morphology.