The effects of sodium ipodate (ORAgrafin) on thyroid function in rainbow trout, Salmo gairdneri.

Immature rainbow trout held at 12 +/- 1 degree were injected intraperitoneally with a fine saline suspension of sodium ipodate (5 mg/100 g body wt) every 3 days. Plasma 3,5,3'-triiodo-L-thyronine (T3) fell to 40% of control levels by Day 1 and remained at about this level for the duration of the study (22 days). Plasma L-thyroxine (T4) level was not altered on Day 1 but was lowered to 50% of control values by Days 7 and 22. Immersion of trout in T4 (2 micrograms/100 ml water) elevated plasma T4 but did not alter the ipodate suppression of plasma T3. Injection of control or ipodate-treated trout with [125I]T4, [125I]T3, or Na131I indicated that in addition to blocking T45'-monodeiodination to T3, ipodate also decreased plasma clearance of T4 and T3 and their removal by the bile. Ipodate did not alter the hepatosomatic index but did depress the hematocrit by 22 days, possibly due to the hypothyroid state. In conclusion, ipodate at a dose of 5 mg/100 g, approximately one-tenth of a lethal dose, is an effective acute and chronic hypothyroid agent to administer to trout.