Thyroid UK

Confused about reasons for change in TSH/FT4/FT3 levels

I’m confused about the significant change in my test results that has occurred in a very short time and would appreciate any comments. I am hypo (not Hashi’s) and have been on Levothyroxine (50mcg) for 12 years.

First set of test results from the beginning of August are as follows:

TSH 1.7

FT4 14.9

FT3 2.9

Second set of test results from mid August are:

TSH 1.5

FT4 17.7

FT3 4.35

The only difference is that I started taking supplements after getting the first results back (vit B12 (1000mcg initially), selenium, zinc, magnesium and vit D). Also, the first sample was taken around 5pm, while the second one was taken at 9am (fasting).

I have several questions:

1)Can supplements have such a dramatic effect, and within such a short space of time?

2)Can the time the samples were taken affect the results so drastically?

3)Is the free T3 level in the second set of results still a bit low, in your opinion?

I don’t think it’s a simple case of a miscalculation at one of the labs, because my symptoms seem to have improved too. At the time of the first test, I was experiencing hypo symptoms (tiredness, depression, hair loss, difficulty losing weight, pins & needles/tingling etc.) and that was the reason for having the tests done (I was sick of being told that my thyroid function is “normal” by the GP). By the time I had the second set of tests done, I was actually feeling much better (tingling had completely gone, mood and energy greatly improved), so this does correspond to the improved levels.

I have decided to keep taking supplements (vit B12 5000mcg, minerals, vit D and a B vitamin complex) and keep a diary of my symptoms. Then get re-tested in mid September and go from there.

Any advice would be gratefully received!

Thank you,


4 Replies

I suspect that you are absorbing your thyroxine better as a result of better health from supplementing, although I am certainly no expert.

I also suspect that if you had had the blood drawn at the same time of day the second time around, your TSH would have been lower. TSH is highest first thing in the morning. Time of day could definitely have an effect on that but I wouldn't expect it to have such a large effect on T3 and T4.

I read somewhere that B12 is important for good communication between the hypothalamus and pituitary gland, which is why some people with low B12 also have an uncharacteristically low TSH. I can't for the life of me find this now though, which is incredibly frustrating! Maybe one of those who know more about B12 will be able to help with that.

If you are feeling better supplementing, it sounds like you are probably doing the right thing. Getting retested is very sensible, especially for vitamin D as you don't want to go overboard there. Try to get tested first thing again :)

I hope your improved health continues :)

Carolyn x


Hi, the difference in your blood tests is not what 'I' would call 'dramatic' but very slight....TSH is usually higher early in the morning and fT4 and fT3 higher, so the second sample being taken at 9am (compared to 5pm with the first) is the major factor here, I would expect the 9am results to show higher fT3 / fT4 and usually higher TSH though your TSH is lower it is only a 'smidge' lower and to be honest if you were to have ten blood tests each 5 minutes apart from the other they would show different numbers of TSH anyway as it fluctuates quite a lot.


Really? I am definitely not an expert, but I thought that the difference in FT4 and FT3 was significant...


sorry it could be because I see hundreds of blood test results so am 'used' to see the variations of these labs values throughout the day.


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