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Endocrine System Control: Axes, Feedback Loops, and Hormone Measurement Methods

This chapter discusses the endocrine system's regulatory mechanisms, focusing on the hypothalamic-pituitary axes and their feedback loops. It highlights the three main axes: Hypothalamic-Pituitary-Thyroid, Hypothalamic-Pituitary-Adrenal, and Hypothalamic-Pituitary-Gonadal. The chapter explains how hormone secretion is controlled through negative and positive feedback mechanisms, inhibitory control, and metabolic regulation. Additionally, it covers various hormone measurement methods, including immunoassays and bioassays, emphasizing the importance of accurate hormone level monitoring.

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Endocrine System Control: Axes, Feedback Loops, and Hormone Measurement Methods

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  1. Chapter 2 • Endocrine system control • axes and feedback loops • Hormone measurement • methods, normal range, fluctuations

  2. Hypothalamic-Pituitary Axes • Three main axes involving hypothalamus and pituitary control much of endocrine system - operate by negative feedback (short and long loops) • Hypothalamic-Pituitary-Thyroid axis (HPT) • Hypothalamic-Pituitary-Adrenal axis (HPA) • Hypothalamic-Pituitary-Gonadal axis (HPG)

  3. Hypo – Pit-Thyroid Axis

  4. Hypo-Pit-Gonadal Axes

  5. Control of Hormone Secretion • Basic or simple control • Negative feedback mechanism • Positive feedback mechanism • Inhibitory control • Metabolic control

  6. Basic or simple control • Limited hormone made and/or stored • Full release on stimulation • Response is limited in magnitude or duration

  7. Negative Feedback Loop • Most common control mechanism in endocrine system • Eg. thyroid releases T3 and T4 • Feedback to hypothalamus (and pituitary) limits releasing signal

  8. Positive Feedback Control • Less common mechanism • Used when signal amplification is needed • Eg. Oxytocin (made in hypothalamus and released by posterior pituitary) during parturition and suckling • E.g. Mid-cycle LH/FSH surge induced by high levels of estrogen

  9. Inhibitory Control • Hormones that are not released until a specific inhibitor is suppressed • Example: • Dopamine inhibition of prolactin release • Somatostatin inhibition of growth hormone

  10. Metabolic Control • Activation of bioactive hormone requires modification (cleavage, de-iodination, reduction) • May occur in target tissue • Eg. 5-alpha reductase converts testosterone (T) to bioactive dihydrotestosterone (DHT) • Aromatase converts androgen to estrogen

  11. Endocrine Rhythms • Hormone levels may fluctuate in response to external stimuli (food, light, activity) • Circadian and longer rhythms also exist • E.g., pulsatile secretion of pituitary growth hormone

  12. Fluctuations • Changes in hormone levels occur normally throughout day (and night) • Accurate measures may require multiple sampling over time • Peripheral blood levels are only one measure; local concentrations may be higher and more relevant

  13. Measurement of Hormones • Hormones present in nanamolar to picomolar range • Very sensitive tests required

  14. Immunoassays • Radioimmunoassay (RIA)– Pure radiolabelled hormone (H*) binds to antibody and Unlabelled hormone (H) from bio-fluid competes for binding and displaces H* • Non-radioactive Enzyme-Linked ImmunoSorbent Assay (ELISA) – Immobilized 1st antibody binds hormone that in turn captures an enzyme-tagged 2nd antibody. • Sensitive, quantiative

  15. Other Hormone Assays • Radioreceptor assays (RRA)s use hormone receptors instead of antibodies • Bio-assays use target cell outcomes to measure biological activity of hormone sample (or biological fluid) • Cells in culture may respond by expressing a known gene product • New science of detecting “endocrine disruptors” in environmental materials

  16. Summary • Axes and feedback loops • Control of hormone secretion • Measurements of hormones

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