While no hormone directly releases pre-formed platelets from storage, thrombopoietin is the key hormone regulating platelet production. It doesn't trigger the immediate release of platelets into the bloodstream, but rather stimulates the bone marrow to create more of them. Platelets are then released into circulation as needed.
Understanding Platelet Production and Regulation
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Thrombopoietin's Role: Thrombopoietin, a hormone produced primarily in the liver and kidneys, is crucial for megakaryocyte development. Megakaryocytes are giant cells in the bone marrow that fragment to form platelets. Increased thrombopoietin levels lead to increased megakaryocyte production and, consequently, more platelets. [Source: Wikipedia, Megakaryocyte and platelet production is regulated by thrombopoietin, a hormone produced in the kidneys and liver.]
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Platelet Release: The release of platelets into the bloodstream is a complex process involving several factors, not solely a hormone. It's triggered by various stimuli like vascular injury, requiring a coordinated response of the body's hemostasis system.
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Other Hormones and Platelet Function: While thrombopoietin is central to platelet production, other hormones influence platelet function. For example, estrogen modulates platelet activity. Growth hormone release-inhibiting hormone has been shown to impair platelet aggregation. [Source: The effects of estrogen and hormone replacement therapy on..., Impairment of platelet function by growth-hormone release-inhibiting...] Furthermore, platelets themselves release various substances, including serotonin, which aids in wound healing. [Source: Serotonin: What Is It, Function & Levels]
Clarification: The Question's Nuance
The original question implies a direct hormonal trigger for platelet release. However, the process is more nuanced. Thrombopoietin governs production, not immediate release. While various substances and signals influence platelet activity and release, no single hormone solely controls their immediate entry into circulation.
