Well, it's been a long year and a bit, but I've finally been told by my GP that I have an underactive thyroid.
I now start on levothyroxine with the dosage 25mcg a day, taking 1 tablet in the morning before breakfast. The dose goes up 2 weeks after to 50mcg and then 2 weeks after that it goes up to 75mcg until - well, the prescription states from two months onwards, so I presume for life.