Iodothyronine deiodinases and the control of plasma and tissue thyroid hormone levels in hyperthyroid tilapia (Oreochromis niloticus)

In fish, as in other vertebrates, the thyroid secretes predominantly 3,5,3′,5′-tetraiodothyronine, which has to be converted into 3,5,3′-tri-iodothyronine in order to bind to a nuclear receptor and exert its full biologic activity

S Van der Geyten; N Byamungu; G E Reyns; E R Kühn; V M Darras

2005

Scholarcy highlights

  • In fish, as in other vertebrates, the thyroid secretes predominantly 3,5,3′,5′-tetraiodothyronine, which has to be converted into 3,5,3′-tri-iodothyronine in order to bind to a nuclear receptor and exert its full biologic activity
  • Two weeks of T4 supplementation had no effect on the plasma T3 and T4 concentrations in tilapia. These observations agree with data obtained in rainbow trout, where 3 days of T4 supplementation did not affect circulating thyroid hormone levels
  • In addition to plasma T3 and T4, the tissue T3 and T4 concentrations, as well as the different deiodinases expressed in these tissues, remained unaffected by feeding the T4-supplemented food, with the liver as the sole notable exception
  • In T4-fed tilapia, the hepatic T4 levels increased substantially, and this was accompanied by an increase in in vitro D1 activity, while hepatic T3 concentrations, as well as D2 and D3 activity, were not affected
  • Despite the difference in the amount of T4 used between the experimental approaches, the observation that immersion in T4-supplemented water increases both plasma and hepatic T3 and T4 levels, while feeding of T4-supplemented food does not, suggests that the T4 ingested with the food is not taken up efficiently from the gut, as has been proposed by several authors
  • T3 concentrations are, on average, 80% lower than plasma T3 levels, while in the other tissues these levels are equal to or higher than the levels found in circulation
  • These results suggest that the lack of effect of T4 supplementation via the food on circulating T3 and T4 might be due to a rapid conversion of T4 into rT3. This hypothesis is supported by our finding that the increase in hepatic T4 is accompanied by an increase in hepatic D1 activity, which in turn is able to increase the conversion of T4 into rT3 via IRD
  • The presented results demonstrate that in hyperthyroid tilapia the increase in plasma T3 levels is reflected in the liver, brain, kidney, gill and white muscle without affecting local T4 concentrations

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