If i lose weight (through dieting), should i visit my doctor to decrease levothyroxine? at how much weight loss should my dosage change?

Confused: Assuming you are taking levothyroxine for hypothyroidism and not weight control then your weight won't affect your dosage. To any great degree.

Perhaps: If you lose enough weight your dose may need to be decreased. The only way to tell is to have your levels checked.

Limited Relation: Persistent low thyroid activity, e.g. Tsh 50-100% above top end of "normal" range (tsh inversely responds to thyroid hormone) sometimes correlates with ↑body fat. Within the nl ; low TSH range there is little reliable relation between thyroid activity ; body fat. Yet thyroid supplementation has long been promoted, by some, to ↑ wt loss; no evidence true or safe. ↓fat unlikely to change thyroid.