Objective: Thyroid hormone modifies cardiac action potentials and outward potassium currents directly and indirectly e.g. through β-adrenergic signaling pathway. We thus examined the expression of six voltage-gated potassium channel α-subunits in the rat left ventricle under hypo- and hyperthyroid status, and tested roles of β-adrenergic signaling pathway in their expressions under both status. Methods: Hypothyroidism and hyperthyroidism were induced by administration of methimazole (MMI) for 4 weeks and by injection of l-thyroxine (T4) to the MMI-treated rats for the last 7 days, respectively. To distinguish the effects of T4 and the β-adrenergic system, propranolol (Pro) was administered to the MMI-treated rats together with T4, and isoproterenol (Iso) was injected to MMI-treated rats for the last 7 days. The mRNA levels of Kv1.2, Kv1.4, Kv1.5, Kv2.1, Kv4.2 and Kv4.3 in the left ventricles were determined by ribonuclease protection assay. Results: MMI treatment induced hypothyroidism and resulted in a significant decrease in the mRNA levels of Kv1.5, Kv2.1 and Kv4.2 (19%, 77% and 61% of control value, respectively; n=6, p4 administration induced hyperthyroidism and cardiac hypertrophy, and it increased the Kv1.5 and Kv2.1 mRNA levels over the control value (212% and 140%, respectively; n=6, p4. In contrast, the Kv1.2 and Kv1.4 mRNA levels increased in hypothyroid rats (161% and 186% of control value, respectively; n=6, pn=6, p4-induced hypertrophy. Iso induced cardiac hypertrophy. Pro or Iso by itself did not alter Kv mRNA levels except for Kv1.2, the message of which was decreased by Iso. Conclusion: Thyroid hormone differentially regulates the expression of Kv1.4, Kv1.5, Kv2.1 and Kv4.2 mRNA levels in the rat left ventricle. This effect is not mediated through β-adrenergic signaling pathway. On the other hand, the reduction in Kv1.2 mRNA level was associated with cardiac hypertrophy induced by T4 or Iso.