Thyroid UK

Low T4 but low TSH too

I am on 100mg of levothyroxin, but my blood tests have confused me. I used to take NDT and always had a suppressed TSH at near zero. I knew that usually happens on NDT and I have expected it.

Early last year my GP persuaded me to try Levothyroxin by itself and I haven't noticed any difference, to be honest, and we agreed to a six monthly blood test. So, after 6 months on 100mg thyroxin my free T4 was 15.8 and TSH was 0.93.

The next six month blood test came back last week and my free T4 was 14.4 and TSH 0.69. My question is why has my TSH gone down still on the same amount of Levo? If anything, surely it should have gone up, especially as my T4 was lower.

My TSH has never been high. Even when I was diagnosed in 2006 it was only 4.85 and my T4 was 12, but I was very swollen with fluid retention (myoedema?).

Your thoughts are appreciated.

6 Replies


TSH fluctuates throughout the day so testing at different times of day will give variable TSH results. See the graph in

What is the FT4 range (the figures in brackets after results)?

1 like

T4 range is 12-22. I always try to get the test done around 9am. I try to be as consistent as possible which is why it seems strange. I was wondering whether my pituitry is also underfunctioning, too.



There's been a small fluctuation in TSH and FT4. TSH is usually lower in the autumn/winter so that will likely explain it. TSH 4.85 when you were diagnosed indicates your pituitary was functioning just fine.


Thank you Clutter. That makes sense. I didn't realise TSH is often lower in the winter. Thats interesting and has put my mind at rest.



Damn, I should have said TSH is higher in the winter! Please don't worry about the reason your FT4 and TSH have fluctuated, the fluctuations in your results aren't at all significant.


With secondary hypothyroidism the TSH result can be below range, low, normal or just above the normal range.

Both FT4 and FT3 will be inappropriately low.

In my own secondary hypothyroid case my TSH had risen to 6.2( just above range)

but my FT4 was 6.5 (12-22) and my FT3 was 2.1 (3.6-6.8)


With those FT's one would have expected to the TSH to be over 100.


You may also like...