Levothyroxine (T4) is used to treat patients with hypothyroidism and may often result in lifelong therapy. Its physiologically active metabolite is tri-iodothyronine (T3). Levothyroxine is also endogenously produced in the body. Since small changes in levothyroxine administration (e.g. change in brand or formulation) can cause significant changes in serum thyroid stimulating hormone (TSH) concentrations, precise and accurate TSH control is critical to avoid potential adverse iatrogenic effects. Till date, the active pharmaceutical ingredient (API) for levothyroxine tablets marketed in India has been sourced from company 1. GSK India proposes to change the source of the API from the existing source (company 1) to to new source (company 2). In view of this change in the source of API, it is essential to determine whether it has any impact on drug product performance based on pharmacokinetic (PK) measures of total serum T4 and total serum T3 of the to-be-marketed formulation of levothyroxine (test formulation from company 2, Treatment A) relative to the formulation currently in the market (reference formulation from company 1, Treatment B).