The Starting Dose of Levothyroxine in Primary Hypothyroidism Treatment. A Prospective, Randomized, Double-blind Trial.
INTEMED. 2005;165(15):1714-1720. doi:10.1001/archinte.165.15.1714.

"Our study supports the recommendation of a full replacement starting dose of levothyroxine of 1.6 μg/kg in healthy adult patients younger than 65 years. Moreover, our study also provides evidence that it is safe to treat patients older than 65 years with hypothyroidism with a full replacement dose of levothyroxine if they have no history of ischemic heart disease."