novel formulations useful for thyroid hormone replacement and suppression therapy are described herein . while dosage formulations containing iodinated thyronines , levothyroxine sodium or liothyronine sodium , as the active pharmaceutical ingredient dispersed in a polymeric solid are known , manufacturing such formulations with acceptable dose content uniformity is difficult due to the high biological activity of t4 and t3 . described herein is the preparation of a single api formulation containing both t4 and t3 in a pharmaceutically effective ratio , wherein the dose content uniformity is the same for t3 as for t4 even if the t3 dose is extremely low . the present invention provides a method for the preparation of an api wherein t3 is uniformly distributed within the crystal lattice of t4 to form a solid solution by cooling suitable solutions of levothyroxine sodium and liothyronine sodium , a pharmaceutical composition containing such an api , and a method to use such an api in the treatment of thyroactive disorders .