SHH and hyperthyroidism: Thus, it seems possible that similar mechanisms (T3 → TRβ/RXRα → HLF → HIF-1α → EPO and T3 → SHH → BMP-4 → BFU-E) may lead to increased erythropoiesis in human and feline hyperthyroidism and may explain the presence of erythrocytosis observed in this disease.