when I first came in contact with paleo, everything seemed to indicate that low-carb/zero-carb is the way to go. And I followed those suggestions precisely.
Unfortunately, when liver glycogen is sparse, T4 to T3 conversion is inhibited. So it seems that long term ZC/VLC inevitably leads to hypothyroidism. (To solve this problem actually foods rich in fructose work best for me, e.g. buckwheat honey; It seems fructose is only toxic in case of fully replete liver glycogen stores)
Now I'm really interested, if there's anybody out there who actually does/did strict VLC/ZC for months and didn't experience any thyroid issues (cold hands, lethargy, light-headedness, ...) - or if some of you also developed hypothyroidism.