You are correct, heterozygous provides some protection from malaria and homozygous causes sickle cell. The heterozygotes are still not normal and are less healthy than normal people, but the negative pressure from malaria is strong enough that it's a net gain.

Thyroid hormones affect regulation of mitochondria and their metabolism, but through signaling. Iodine isn't directly part of metabolism. That may have been what you meant, just wanted to be clear.