Thyroid UK
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Changing from T3 to T4

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.

5 Replies


I was switched from 60mcg T3 to 200mcg Levothyroxine after RAI. I was very over replaced with undetectable TSH, FT4 35 and FT3 >7.0.

It takes 7-10 days to absorb Levothyroxine. I would continue taking 60mcg T3 until day 5 and reduce to 30mcg days 6 and 7 and then stop T3.

Levothyroxine only made me very unwell and FT3 dropped below range. I'm good on T4+T3 though and I prefer the combination to T3 only.


I am not a medical professional and this information is not intended to be a substitute for medical guidance from your own doctor. Please check with your personal physician before applying any of these suggestions.

1 like

Thanks very much for this info. Good to hear from someone who has been there. I'll be interested to see how my T3 to T4 only experiment progresses. There does appear to be something wrong with the medical system regarding thyroid hormones. Have you some analysis regarding how many different people report that they have an issue with T4? Even allowing for the self-selection factor, it would appear that there are sufficient examples of an issue that the medics/NHS need to address. Thanks again.



It is widely reported that up to 15% of patients don't do well on Levothyroxine only but I don't have links to substantiate this.

These studies below show that some thyroidectomised patients do not respond well to Levothyroxine only and may need T4+T3 combination therapy.


Thanks again, I'll let you know how I get on. Interesting times!


Hi there, I know I am late with replying to this post but wanted to know how did you go on?


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