Regulation and Measurement of Thyroidal Status in Fish

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Abstract
The teleost thyroid secretes L-thyroxine (T4) under the control of pituitary thyroid- stimulating hormone (TSH), regulated by predominantly inhibitory hypothalamaic neural pathways. Plasma T4 exerts negative feedback control at both pituitary and hypothalamic levels, but the set-point changes, altering plasma T4. A commonly-held view is that thyroidal status (availability of hormone to receptors) is driven by the brain- pituitary-thyroid axis. However, plasma T4 levels are often exceeded by those of 3,5,3'- triiodo-L-thyronine (T3), a tissue derivative of T4 with greater receptor affinity. T3 availability 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 physiological state stabilize plasma T3 despite T3 or T4 challenges. Furthermore, plasma T3 can be depressed without change in plasma T4. Thus thyroidal status in teleosts 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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