Thyroid UK
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I Want To Replace Some T4 with T3 But Need To Know How Much - Can You Help?

My Endo seems to think I am under replaced by around 25mg on T4, although on the five different Formula's spreadsheet that Helvella did Im averaging out at around 30/40mg under at the moment. Im having more blood tests next week and providing the results are much the same I want to suggest to him a trial of T3 so that instead of increasing with T4 that I use T3 instead as I know he wont go for a total switch to T3.

Can somebody please tell me what the T3 equivalent would be for the following amounts of T4:

a) 25mg

b) 30mg

c) 35mg

d) 40mg

e) 45mg

f) 50mg

Could you also please answer the following:

1. Is it is best to take the above amount of T3 once a day and what time of the day that should be. If not once, how many times a day?

2. Can you take the T3 with the T4 (which is taken first thing in the morning).

Thank You so much in advance and any other advice would be appreciated.

8 Replies

no one can tell you anything about the above and no it's not me being 'funny' but it's the truth, we are all different and only by trial and error and hopefully a doctor helping too you can find what works for you.

I take all my 160mcg of T3 at once when I wake up in the morning, some people would be 'floored' by my approach, I would be 'floored' if I took small doses of T3 or if I divide them.

it is really hit and miss most of the times


OH! i thought Id seen on here some general rough guide about replacing T3 with T4?


well, in 'theory' most cite T3 being around 5 times stronger than T4 so 25mcg of T4 is 5mcg of T3, however how it will affect you it is to be discovered :)

for example, if the above is true then I am taking the equivalent of 800mcg of T4, if I was to take 800mcg of T4 I'd be 'dead' I think HAHAHAHA


Ok I understand. I had sort of worked it out that on a rough estimate that if I had needed to up my T4 by 25mcg then I would have started on 5mcg and if it was closer to the 40mcg then it would have been 10mcg of T3.

The only reason I was asking was so that I could be a step ahead so to speak when I make my suggestion.

TBH there is no way you could split a 5/10mcg dose in any case really other than nibble at it.

Could somebody just advise if it is best to take it in the morning at the same time as the T4 and if not what time would be best. Thanks. :-)


you can take T3 together with T4 that is not a problem, as for what time to take your T3 you will find out what works for you, however I can say that in the initial period most people tolerate it better if taken in the morning and if they split doses to take further doses 4 hours or so apart with latest dose no later than 5pm (for most, but like I said some others feel better take it in the evening, sorry am not being 'ackward' is that really there is not one answer)


I started my daughter on T3 two years ago (she was 22) I did the following, completely stopped the T4 for 2 weeks to get it out of her system, there was no effect of not taking T4 as it stays in your system for ages. My daughter was born without a thyroid & when she was 2 year old they wanted to scan her neck to see if she had some thyroid gland so they stopped all her thyroid medication which was T4 for 14 weeks before the scan & she was OK so stopping for 2 weeks was not a problem for my daughter. After the 2 week break I started daughter on 10 mg of T3 twice a day, 6 hours apart for one week, the first dose as soon as she woke up, the following week doubled the dose to 20 mg twice a day 6 hours apart, then each week increased by 10 mg. She currently takes 100 mg of T3 split into 2 doses & recently I added 25 mg of T4. Within one week of being on T3 only the change in my daughter was amazing. However she is currently not doing so well but this may be her missing the tablets when I'm not there to ensure she takes them. Breaking them in half is easy despite there tiny size, don't bother with a pill cutter or knife just hold the tablet between your fingers & snap in half across the scored section, if one half is bigger than the other it matters not as long as you take them on the same day.


Hi all,. Totally agree that there is no one answer for how to take your meds. For example, I started taking my T4 at night as I know liver is more active at night and thought it would help my conversion to T3. Not sure if it works entirely but its what I do. I also take quarter a T3 pill 3 times a day (with an oj for sugar to prevent me getting a bit of a shake on) though I started with just taking a quarter in the morning. Always with food to help slow absorption too.

I am on 100mcg levo and take about 18mcg t3 daily. It has been the best thing - was on a horrible downward spiral otherwise. Have an appt in June to speak to endo about getting T3 prescribed....I do not think it will go well since I was already ordered out of her office with my symptoms being told they were nothing to do with thyroid.

Good luck!


Originally it was thought that the ratio of T4 to T3 was 5:1. So 50mcg of T4 would be equivalent to 10mcg T3.

Another more recent study suggested that the ratio is actually 4:1 or 3:1.

I suspect it depends on the individual patient. My endo suggested a ratio of 3:1 at my last appointment but I've actually needed to take a little bit less than that.

By the way you can get 5mcg T3 tablets but the endo or GP may not have this information in their "medical bible". As long as it's on the prescription any pharmacy can get it. I did manage to split mine, but it was fiddly so in the end I did just nibble it :-)

Totoro x


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