I have an under-active thyroid and fibro.
I've been aware of links between the two.
today there is a poll asking if anyone has any of 8 conditions as well as thyroid problems.
out of 62 replies 49% have fibro - almost half! so I'm assuming it's 30 out of 62.
It also worth noting how many people on the thyroid forum report being un-diagnosed for some time, despite having many of the symptoms, due to "normal" or "just under" results - including myself.