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Regulation and measurement of thyroidal status in fish
Eales, J.G. (1995). Regulation and measurement of thyroidal status in fish. Neth. J. Zool. 45(1-2): 175-180
In: Netherlands Journal of Zoology. E.J. Brill: Leiden. ISSN 0028-2960, more
Peer reviewed article  

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Keywords
    Marine; Brackish water; Fresh water

Author  Top 
  • Eales, J.G.

Abstract
    The teleost thyroid L-thyroxine (T4) under the control of pituitary thyroid-stimulating hormone (THS), regulated by predominantly inhibitory hypothalamaic neutral pathways. Plasma T4exerts negative feedback contol at both pituitary and hypothalamic levels, but the set-point changes, altering plasma T4. A commonly-held view is that thyroidal status (availibility of hormone to receptors) is driven by the brainpituitary-thyriod axis. However, plasmaT4levels are often exceeded by those of 3,5,3'-triiodo-L-thyronine (T3), a tissue derivative of T4 with greater receptor affinity. T3 availibility is determined by a T4 outer-ring deiodinase (T4ORD) which generates T3, and by a T3 inner-ring deiodinase (T3IRD). Both enzymes are regulated peripherally, and for a given phisiological state stabilize plasma T3 despite T3 or T4 challenges. Furthermore, plasma T3 can be depressed without change in plasma T4. Thus thyroidal status in teleost may not be driven by the brain-pituitary-thyroid axis, but may rely instead on peripheral T3 regulation. This has implications for measuring thyroidal status.

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