No - not Free T4 - but room temperature!
Today's advice is:
Staying warm by heating your home to at least 18°C can be crucial to staying well.
I'd hazard a guess that ordinary indoor temperatures are often in a range of something like 12 to 22 - so numerically very similar to many Free T4 ranges. But the health promoters can clearly see that many of us need 18 (towards the top of the range) on a thermometer. And at the same time claim 12.1 as being adequate for Free T4.
Time to update it:
Staying well by raising your Free T4 to at least 18 can be crucial to staying well.
(Yes - well aware that not everyone would want that level of FT4, that T3-takers might be shocked, etc. Just meant to highlight the ludicrous nature of medical advice and practice.)
I imagine suggesting your GP reduce their office temperature to 12 - because it is in range - would go down very badly. They'd blame patients for needing it much warmer.