Thyroid hormone (levothyroxine) replacement via the respiratory route by inhalation: in vitro exploratory studies.

Objectives: To conduct proof of principle studies that will enable development of noninvasive (respiratory) delivery systems for levothyroxine (T4).

Methods: Preformulation (solubility, stability), formulation and biopharmaceutical (in vitro absorption, transport, gene expression) studies were conducted. Calu-3 cell line was used for permeation studies.