Why is iodine deficiency no longer a common cause of hypothyroidism in the united states?

Can someone give me one specific health problem (within unhealthy eating) and explain it. !

If an online school program (i. e k12, ecot) assigns me a computer, would it be ok to use social media on it? would i get in trouble? i am not really sure from reading their terms and such.

The consists of a mixture of gases and particles, including oxygen, nitrogen, hydrogen, carbon dioxide, water vapor, and dust.