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How long for Free T4 to rise?

On Jan 11 on dose 88mcg Synthroid my TSH was 0.88 (0.32-5.04), Doctor refused to run T4/T3. Since then I got a second opinion to raise my dose to 100mcg and will be on it for 18 days when I get my blood work done in a couple days.

The new walk in doctor wrote in a few tests (iron, B12, etc) but also TSH so it will get tested anyways. As he wrote it in pen, I will write in Self Pay T4/T3 myself as I need to know where they stand.

I wish I knew where my T4/T3 levels were on the 88mcg but this is best I can do. So yes, I know TSH takes time to dial in (6-8 weeks) but will the Free T4 be accurate for the 100mcg Synthroid for 18 days? At the least it can tell me if I’m going in right direction. Meanwhile I am searching for a long term doctor before needing to rerun labs and adjust dose in 3-4 weeks (will be on 100mcg for 6 weeks then). Thanks.

14 Replies

but will the Free T4 be accurate for the 100mcg Synthroid for 18 days?

I doubt it. It takes about 6 weeks for the full effects of Levo dose changes to be noticed, and this may well apply to test results too.


Fair enough! Do you think the T4/T3 will at least give me a good idea of what they were on the 88mcg (6 weeks), or I guess between that and 100 dose. I just need to know what the levels are now so I can compare it to how I feel.


The results might be slightly higher than on 88mcg but not accurate enough for the 100mcg dose. As you are having to pay for the tests yourself, would it not be better to wait until you've been on the 100mcg dose for 6 weeks?

1 like

I am amazed by this, but it kind of makes sense.

Left column day 0 through 48. Peak concentrations of 933 and 1060 respectively for 88 and 100 mcg. Once you start adding 100 instead of 88, it takes 48 days to reach full concentration. But look at day 18...it is 98% of the new concentration. And you have gained 84% of what you will gain. I know this probably falls into the category of too much information, but I thought it was interesting. When I first saw the question I was like 18 days?, give me a break. lol

0 933 1060

1 945

2 956

3 966

4 975

5 983

6 990

7 997

8 1003

9 1008

10 1013

11 1018

12 1022

13 1026

14 1029

15 1032

16 1035

17 1037

18 1039

19 1041

20 1043

21 1045

22 1046

23 1048

24 1049

25 1050

26 1051

27 1052

28 1053

29 1053

30 1054

31 1055

32 1055

33 1056

34 1056

35 1057

36 1057

37 1057

38 1058

39 1058

40 1058

41 1059

42 1059

43 1059

44 1059

45 1059

46 1059

47 1059

48 1060


I wouldn't have though it would be enough time, but looking at the half-life it takes six weeks to reach full serum concentration, but going from 88 to 100mcg at the 18 day mark you are at 98% of what the new concentration will be for T4.


That may be what happened to you, it doesn't mean to say it will be the same for everyone.

It's generally advised to wait 6 weeks to retest, I often say 6-8 weeks, another member says 4 weeks. I do frequent tests because I am tweaking Levo/T3 doses. I have learned, over the past 2 years, that for me personally 6 weeks isn't enough, 8 weeks is best for me to see exactly how the dose change has affected my results.

One size doesn't fit all.


This was not my experience, it is simply based on the 7 day half life of levo.

How it affects TSH etc is another ballgame. It's just interesting how slowly it mounts toward the end of the six weeks.


Yes, the FT4 level will normalise pretty quickly but that's not the point. Before you started taking T4, your body conversion to T3 was greatly altered from health. What takes time is for the new rate of body conversion of T4 to T3 to stabilise when you start T4 therapy. This requires some days if not weeks for conversion to settle and give you the new conversion rate suitable for you. It may not be, and thats why further dose tweaking might be necessary. This is one of the reasons for a delay in symptom resolution after starting T4.


Diogenes, why is it that, as reported by some people, that as the dose of T4 goes up the fT3 goes down?


I suppose in that case, for some reason T4-T3 conversion increases. It must be in response to inadequate FT3.


I appreciate the replies, very educational! I decided to simply have the TSH run, it’s not as if I will revisit dose adjustment yet. Best to save that $40 for retesting in 3-4 weeks and run the Free T4/T3 then.

I’ve simply been becoming anxious as of late, trying to rush things and get as much information about my health as possible. Especially with this IBS and gastrocolic reflex every morning after breakfast, interfering with my ADHD medication efficacy. But best be patient!


So got my lab results today, fasted for 10 hours and took Synthroid 22 hours prior. 18 days on 100mcg.

TSH 0.44 (0.32-5.04)

Previous results:

0.88 on 6 weeks 88mcg and 0.44 (same!) on 6 weeks 75mcg.

B12 692 (153-655)

Ferritin 105 (24-444)

ALT 126 (<50) AST 54 (<36)

TPOab been in range for years now (last done few months ago) along with TGab.

Will need to further check the liver, I haven’t drank in 6 months, have borderline fatty liver (ultrasound done year ago) but ALT never rose past 60 (<50) even when I was off thyroid meds.

Will retest TSH along with T4/T3 at 6-8 week mark on 100mcg so in 3-5 weeks. May check my Vitamin D soon as well. 5 months ago it was 84 (75-150) while taking D3 10,000 a week, since then taking D3 12,000 once a week to bring to 100.

Funny that my TSH has been the same on 75, 88 and 100mcg Synthroid. I wonder how accurate T4/T3 will be for dosing as the liver would decrease them (chicken or egg).


A question: you can write-in your own tests in pen, and pay for them? I was clearly informed that you can’t do that in Canada, you cannot pay for your own testing here. How are you able to do this? I’d love to just choose my own tests, but I was told I cannot do so.


No you can’t but the doctor can write self-pay on the lab requisition paper. Though often a doctor isn’t aware that a certain test won’t be run by the lab even if written on the paper, so that’s when I just might write self pay myself.


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