Lithium is a medicine used for treatment of certain disorders of the mind (psychiatric disorders). Lithium can cause slowing of the heart rate (bradycardia) as well as the conduction within the heart and occasionally produce AV block (atrioventricular block). This can sometimes be severe enough to need a temporary pacemaker to give artificial electrical signals to the heart. Another mechanism by which lithium can slow down the heart is by hypothyroidism. Lithium can cause reduce function of the thyroid gland known as hypothyroidism. When the thyroid hormone level in the blood is low, heart rate can come down. All these changes are reversible by discontinuing lithium. If the slowing of heart rate is significant enough to become dangerous, stopping of lithium and substituting it with another medication is advisable.