There are no official guidelines to assist in answering this question. I will search for relevant research papers instead.
The FDA Drug Labels for thyroxine (PO) do not address this question.
From the Research
Half-Life of Thyroxine (T4)
- The half-life of thyroxine (T4) is not explicitly stated in most of the provided studies, but it is mentioned that the treatment with levothyroxine will start for all patients with a reference dose of 100 µg, which can be adjusted by the treating physician to a more safe and appropriate dose for the individual, and that the final titration target together with the individual thyroxine half life can be calculated 1.
- One study mentions that the plasma half-life of D-thyroxine (DT4) is less than 1 day 2.
- Another study states that synthetic levothyroxine (L-T4) has a long half-life, which allows for once-daily administration, and that the occasional missing of a tablet causes no harm 3.
- The long half-life of levothyroxine is also mentioned in another study, which states that dose adjustment should only be considered after 6 to 12 weeks, given the long half-life of levothyroxine 4.
- However, the exact half-life of T4 is not provided in any of the studies, but it is implied to be around 6-7 days, as a new equilibrium is reached after approximately 6 weeks, implying that laboratory tests should not be done earlier 3.