Notably, D3 is frequently overexpressed in malignant cells, which raises the question “why are circulating TH levels not always perturbed by neoplastic cells?” It is likely that, in most conditions, due to the low D3 expression in tumor cells, and more importantly, to the efficient homeostatic hypothalamic-pituitary-thyroid axis, paraneoplastic D3 is unable to perturb significantly the circulating level of TH. The gene discussed is DIO3; the disease is neoplasm.