Ray Peat said:"Since T3 has a short half life, it should be taken frequently. If the liver isn't producing a noticeable amount of T3, it is usually helpful to take a few micorgrams per hour. Since it restores respiration and metabolic efficiency very quickly, it isn't usually necessary to take it every hour or two, but until normal temperature and pulse have been achieved and stabilized, sometimes it's necessary to take it four or more times during the day. T4 acts by being changed to T3, so it tends to accumulate in the body, and on a given dose, usually reaches a steady concentration after about two weeks.
An effective way to use supplements is to take a combination T4-T3 dose, e.g., 40 mcg of T4 and 10 mcg of T3 once a day, and to use a few mcg of T3 at other times in the day. Keeping a 14-day chart of pulse rate and temperature allows you to see whether the dose is producing the desired response. If the figures aren't increasing at all after a few days, the dose can be increased, until a gradual daily increment can be seen, moving toward the goal at the rate of about 1/14 per day"