Oh, excellent reminder, I'm due another blood test for this next week! I was diagnosed with this last July, and the doc's have been trying to sort out my dosage since.
The weirdness (I think, I don't pay much attention) is that I don't actually have an underactive thyroid, but I produce thyroid antibodies that counter the thyroxine I produce. And it seems these antibodies are awfully unreliable. The results of my last test showed a lower thyroxine level than the previous test, even though the prescription dosage had been raised. And I've also had a wee bit of an overdose too, which was interesting.
I'd heard it was more common in women than men. Certainly both my grandmothers had it and both my sisters have it, whereas I'm the only bloke in the family known to have it.