Thyroid UK
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T4 to T3 conversion question

Sorry to ask but I had some bloods done with medichecks which came back yesterday.

TSH 11.14 (0.27-4.20)

Free thyroxine 13.56 (12.00-22.00)

Total thyroxine 72.5 (59.00-154.00)

Free T3 3.96 (3.10-6.80)

Antibodies all high, have Hashis, I understand those results

(I am on 50mg of Levo which I started only 3 days before taking the bloods. Followed instructions to do an early morning test and take levo afterwards not before etc.)

I'm trying to work out how to check I'm converting T4 to T3 sufficiently when I only take levo rather than levo and T3 as some people do. I think it's:

Free thyroxine (13.56) divided by Free T3 (3.96)

This gives a result of 3.42. Does that mean I'm converting well or not, please?

Many thanks in advance for any advice

5 Replies


It was premature to do a blood test only 3 days after starting 50mcg. It takes 6-8 weeks for thyroid levels to respond to a dose of Levothyroxine.

You need to wait until your TSH is 1.0 or lower before you start trying to work out whether or not you are a good or poor converter. Low TSH with FT4 high in range and FT3 low in range indicates poor conversion.


Drat! Thanks Clutter- I'm seeing an endo and wanted the results for my first visit as GP doesn't test T3. Presumably the results won't be of any use to him either!



It takes 7-10 days to absorb Levothyroxine so the results reflect your unmedicated thyroid levels but not a response to 50mcg. You might as well show the endo anyway.

The T3 result is probably only within range because the high TSH is flogging your thyroid to produce T3 and is also flogging T4 to T3 conversion.


Thank you Clutter. Much appreciated. Still trying to understand everything but learning lots. Thank you.

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Your conversion looks fine, but you are undermedicated (if you have only just started levo it won't have kicked in yet as it takes about 6 weeks) - TSH is far too high (needs to be under 1) and Free T4 is low in range - needs to be in top quarter of range (at least 19). Therefore your Ft3 is also low in range.

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