FT3 range

Hello all.  Just a short question.  If you are taking T4 and T3 (or T3 only for that matter) is it at all possible to obtain an accurate FT3 reading bearing in mind the short half life of T3?  If it is possible, how much time to leave since last T3 dose and blood test?  Or is it impossible, meaning that you must adjust medication based on hyper or hypo symptoms alone?  I have read that a suppressed TSH is acceptable as long as the FT3 is in range and not over.  But how can you be sure it's in range?  Duh......

Thanks for any input.

13 Replies

  • Leave about 6 - 8 hours from your last dose, assuming you are taking T3 medication twice daily.  If it's longer than 8 hours add a bit to your fT3 figure and reduce your TSH a bit more.  I'm saying 'a bit' because it would be pseudo-science to try and be more accurate.  TSH will be suppressed if you take sufficient T3 to suppress it.  You might need this amount so symptoms are very important, also monitor for too much by checking pulse and looking for fine hand tremor.

  • Thanks jimh, looks like it's more symptoms than anything else.

  • Have you seen this graphic? tiredthyroid.com/blog/2015/...

    After looking at it, I think my answer to your question is no, you can't ever get an accurate FT3 reading.  So yes, you have to use symptoms.  If you have the time and money, you could try testing 4 hours after your dose, 8 hours, and 24 hours.  But I think you'd just recreate the graph in that post.  However, you'd then be able to see which level correlated most with how you felt.

  • HIFL, I think that your reply together with the excellent link and graphic you provided are the definitive answer to my query.  Thank you.

  • Unfortunately these are early studies when thyroid hormone assays were of limited capability.  More important they were only able to measure total T3 and not free T3.   Free T3 gives a misleading figure soon after ingestion but then responds as expected for a half live of a day or two.  I don't have the reference to hand, sorry.  TSH responds more slowly and so gives a better indication, at least of the T3 action on the pituitary.

  • I have been wondering this too, as I take my t3 and t4 in one dose, which is always 24 hours before a test. So my ft3 is always pretty low, but I don't know if it's too high at other times. My pulse is very low and I feel cold quite a lot so I figure I can't be over medicated?! 

    How are you feeling?

  • Mountaingoat, from the responses above to my original question, I think we can conclude that once one is taking additional T3 one can no longer rely on a blood test to establish whether one's FT3 is within the reference range.  It appears one has to go by symptoms: tremor, pulse, temperature, heartbeat, etc....

  • Hmm if only our endos agreed!

  • Though having read that article in more detail, the potential swing between being quite over medicated and normal or under within 24 hours does concern me a bit. Do you take your t3 once a day?

  • Mountaingoat, I take 12.5 mcg 4 times a day, 6.30am (with 75 mcg T4), 11.30 am, 4.30 pm, 9 pm.  Still have joint/muscle pain, sometimes cold hands and feet.  I personally think I need another dose increase, however my TSH is suppressed, so we'll see what the Dr has to say about that.....

  • Have you ever tried taking it in fewer, larger doses?  From what I've read, some people feel better doing that.  I hope your Dr will listen to you.

  • I'm kind of frightened to do that....I'm 77 yrs old!

  • Aw bless you! Well if the approach generally ain't broke then don't fix it! 

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