The problems such as breaking down or squeaks might be caused by acid reflux from the stomach. (Acid reflux gets so common as we get older.) Sometimes when I was singing, by vocal folds would suddenly 'slam shut,' and I was unable to make a sound.

Something said on the Mudcat led me to suspect acid reflux. I started putting dry milk powder in my coffee (neutralizes the acid) and my voice hasn't slammed shut while singing since.

I do notice acid reflux at other times, and I drink a glass of lukewarm water. Again, this is a helpful suggestion that I learned on a thread right here.

I have a friend who had a thyroid problem. She mentioned to the doctor that she was cutting back on talking, and his response was "Whatever for?" As a result, I suspect that unless your thyroid has been allowed to get in awful shape, that there is no connection between the thyroid and the voice.

Fits of coughing - alas, I find that post-nosal drip can start me coughing in the midst of a song. Maybe see an allergist or eye-nose-throat doctor. Don't give up singing without a fight!