I've had a similar problem with my voice.  My singing voice became almost
non-existent after going hypothyroid.  It has gotten better on medication.
 However, it's still not the same as it use to be.  When I first started on
meds, my voice would come back beautifully and then disappear again.  I'm not
normal yet, nor is my voice, but I'm pretty darn sure it's because of my
thyroid.

Keep in mind that all of the muscles in your body, including your vocal
chords, could be impacted.

Best to you!

Cherise