Please understand that I was speaking in generalities. Keep in mind that I'm not a health-care professional... just your above-average student of biology.
But, as a layman, I wouldn't consider it a far stretch. The thyroid system's main purpose is to regulate growth, metabolism, and energy levels.
As we all know, smoking cigs affects all 3 of those... so it'd make sense to think that suddenly removing chemicals that the body has come to rely on taking in (and producing certain levels of chemicals to adjust for) would have an adverse affect - at least immediately - on the systems that regulate those levels. That's a lot of the physiology of what withdrawal symptoms are.
Hundreds of chemicals in cig tobacco smoke... so lots of variables there, ya know.
The good news is that the body is fairly adept at adjusting to suit its environment (as I mentioned earlier, it tends toward a state of homeostasis), and - just like it adjusted in to compensate for whatever in the first place - provided it's still young enough, it should generally be easy to adjust back to where it was before it started taking in hundreds of poisons.
Maybe, with your parents, their systems were just too old, and, perhaps already had genetic markers of thyroid issues working against them.
Regardless of what the case is, though, as you inferred, I'm sure their bodies are much happier without those chemicals in them.