Which hormone primarily stimulates red blood cell production in the bone marrow?

Erythropoietin is the primary hormone responsible for stimulating the production of red blood cells in the bone marrow. It is produced mainly by the kidneys in response to low oxygen levels in the blood. When oxygen levels drop, erythropoietin is released into the bloodstream, where it travels to the bone marrow and promotes the differentiation and proliferation of erythroid progenitor cells into mature red blood cells. This process increases the overall hematocrit and the oxygen-carrying capacity of the blood.

While testosterone can have an indirect effect on red blood cell production by stimulating erythropoietin production and enhancing erythropoiesis, it is not the primary hormone involved in this regulation. Cortisol has several physiological roles, including some effects on metabolism and inflammation, but it does not play a direct role in stimulating red blood cell production. Thyroid hormone influences overall metabolism and can affect various physiological systems, but it is not specifically responsible for red blood cell production.