Thyroxine and protein-bound iodine concentrations in plasma of the pacific hagfish, Eptatretus stouti (Cyclostomata)

1. 1. Plasma levels of thyroxine (T 4) were determined by competitive protein-binding analysis and compared with plasma concentrations of protein-bound iodine (PBI). 2. 2. Measurements were made on samples from normal, hypophysial-Lesioned and sham-operated fish. 3. 3. In normal animals, which had been held in the laboratory for 3 weeks, the measured T 4 values ranged from 2 to 3 μg/100 ml of plasma and PBI levels varied from 5 to 8 μg/100 ml. 4. 4. T 4 concentrations were significantly lower in both the hypophysial-Lesioned and sham-operated fish which had been maintained in the laboratory for 18 weeks. 5. 5. PBI levels tended to be lower in the experimental fish than in normal animals, but the differences were not significant. 6. 6. In both normal and experimental hagfish, thyroxine iodine comprised less than one-third of the plasma content of organic iodine.