Half life of levothyroxine after increase - Thyroid UK

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Half life of levothyroxine after increase



I have been on 75mcg Levothyroxine and 10mcg Liothyronine for the last 6 months. I recently increased my levothyroxine to 100mcg 6 days ago but found out I need to get bloods taken again. I would like my levels to be what they were before I made the increase (even though i'll probably feel rubbish again).

Should I decrease my dose to the original 75mcg levo and wait until the additional is out of my system? Would I be right in thinking 6 days of extra 25mcg a day would take about a month to leave the system? (Total extra being 150mcg and half life being one week). Or if I took 50mcg instead of 75mcg would it cancel itself out in 6 days?

Any advice would be much appreciated.

8 Replies

Your question is a bit complicated and I don't know the scientific answer. If it were me, I would take the dose that I think is correct for me and retest in 6 weeks time when things have stabilised enough to get a reliable result. Then you can adjust the dose when you have the result.

A lot of people on this forum seem to prefer to increase the liothyronine to 15 or 20 mcg rather than increase levothyroxine but as I don't take T3 I can't comment from experience.

weezles in reply to Nanaedake

Thank you for your reply Nanaedake. I increased the t4 as consultant said that's the only one he would change. I expected an increase based on my results but alas no. I have to go see him and get more bloods taken.


If you wish to reduce dose again, I think it takes about six weeks for levo increase to return back to the previous dose.

weezles in reply to shaws

Thanks Shaws. I did think it would take a wee while.

Suppose you usually took 100 ug T4 regularly and consistently. Then your FT4 resulting would reflect that constant dose. Now suppose you upped the T4 to 150 and kept this level long enough to gain the new higher FT4 level. Now lets go back to 100ug consistently. You have initially a FT4 corresponding to 150 ug T4. After a week, half of the extra 50 will have gone and after two weeks, half of the remaining excess (i.e 25%). After a month, the excess will be down to 6% (effectively back to the 100 ug square one). So in effect one month suffices for you to regain the FT4 typical of 100 ug.

weezles in reply to diogenes

Thanks Diogenes. That what I thought but it seemed quite complicated. I have learned my lesson not to increase my t4 dose myself! Must wait for endo... (or increase t3! 😉)

rosemarie in reply to weezles

I would increase t3 and decrease t4

weezles in reply to rosemarie

Yes I'll know in future. I increased the t4 as consultant said that was the only thing he'd change and I expected an increase based on my results (how naive of me!) Instead more bloods to be taken - and I've just messed them up with the t4! 😕

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