Finally agreed with Doc to try to change from T3 to T4. Currently on 60 micros of T3 daily and Doc has been advised by the edno that I should swap to 300 micros T4. (Don't have a thyroid anymore, was removed 30 years ago due to cancer, and need TSH suppressed due to risk of it returning). I know about half lives of both T3 and T4, but am not sure how long T4 takes to be absorbed and converted to T3. Doc suggested that I should just stop T3 one day and start T4 the next. But unsure how long T4 takes to kick in - assuming it does. Anyone have any hard facts? Have plenty of T3 pills in reserve in case T4 doesn't work! So can reduce T3 intake over a few days while T4 kicks in if necessary. But would like to convert to T4 as it is easier to get hold of - not to mention denying any more £ going to Mercury pharma.