We found causal associations between adiposity and proteins (including IL-6 receptor and transferrin receptor) on subcortical brain iron.ConclusionsWe identified causal effects for liability to type-2 diabetes, depression, and other conditions, on subcortical MRI-estimated brain iron, but not to Alzheimer's disease, supportive of dementia as a consequence of brain iron deposition, not a cause. The gene discussed is TFRC; the disease is depressive disorder.