Low TSH, low FT4, any ideas why?

[pennyred]

Thyroid experts, any thoughts would be much appreciated please. I was on 100 of thyroxine and tested with TSH less than 0.01, FT4 outside upper range. It was reduced to 75, and after 6 weeks TSH was 0.22, FT4 15.3 (12-22). I'm now a week into 100 3 days a week, 75 the others. Not feeling brilliant - haven't been since dose reduced. Why would the FT4 drop so much with a fairly low TSH? Baffled! Am ttc.

Remember that the NICE guidelines say your TSH should be under 2.5 if you are TTC or PG. That's for starters.

Next question. Did anyone think to test your FT3? Hmm. Thought not. Sometimes people don't convert thyroxine (T4) into the active hormone (T3). This means that the TSH is low because the body recognises that the T4 is there, but the FT4 is high because it's just going round your system instead of being converted into something the body can use.

Alternatively, your FT4 might have been just a tad over range and you might at that point have been on the right dose. A suppressed TSH is not an issue if everything else is within range.

You did get your bloods done first thing in the morning, fasting and without taking your thyroxine, didn't you? Any other way is likely to give a skewed result.

Otherwise, you've dropped from 100 to 75, your TSH is coming up and your FT4 has fallen, which is what I would expect.