What's the main difference between raynaud's disease and hypothyroidism?

HypoT may cause Rayn: When a patient has raynaud's phenomenon (vasospasm leading to poor circulation in the fingers and toes) his doctors will look for the common causes such as autoimmune disorders, hypothyroidism, etc... If no medical condition is found that is causing the symptoms, then his disorder is called "primary raynaud's" or "raynaud's disease". Hypothyroidism is a disease that can cause raynaud's phenomenon.

Different disease: They are very different. Hypothyroidism is an endocrine/ metabolic disease, often caused by an autoimmune condition called hashimoto's thyroiditis. Raynaud's is a vasospastic condition sometimes from autoimmune illness like lupus. What they share is trouble with cold. Raynaud's have very poor reactions to cold, spasms in blood vessels. Low thyroid also causes cold intolerance, by low metabolism.