Came across this interesting article (abstract) which suggests, on the basis of rabbit studies, that calcium channel blockers (e.g. for high blood pressure) may interfere with levels of T3 and T4.
I also note that the calcium channel blockers can inhibit uptake of T3 and T4 in rat cardiomyocytes - see
ajpheart.physiology.org/con...
My wife has taken Amlodipine (a calcium channel blocker) for many years. After a hemithyroidectomy, she can only function by taking 40mcg T3 daily. You have to wonder whether the Amlodipine is at least part of the problem.