KINETIC-ANALYSIS OF THYROID-HORMONE SECRETION AND INTERCONVERSION IN THE 5-DAY-FASTED RAINBOW-TROUT, ONCORHYNCHUS-MYKISS

Estimating the 3,5,3'-triiodothyronine (T-3) thyroidal secretion rate and the rate of extrathyroidal conversion of thyroxine (T-4) to T-3 ar e two difficult and important quantitative endocrine system problems i n vertebrates. To address these questions in fish, two thyroid hormone tracer studies were modeled and analyzed, based on data from two grou ps of rainbow trout maintained at 12 degrees and fasted for 5 days. Th e data consisted of three time series: plasma concentrations of radioa ctive I-125-labeled T-3 (T-3()) following T-3(*) injection, and both labeled T-4 (T-4()) and T-3(*) following T-4(*) injection. To facilit ate model parameter estimation plasma volumes were determined independ ently by injection of labeled bovine serum albumin. The T4 injectate was contaminated by an unknown amount of T-3() and this was considere d an additional unknown. A six-compartment model was formulated in ter ms of 13 uniquely identifiable (quantifiable) parameters, which were e stimated simultaneously from the three data sets using a sophisticated optimization algorithm built into a new model-fitting software packag e called FITMOD. The rates of interest, plus other kinetic indices, we re estimated successfully using additional analysis. We found that the thyroid gland secreted 0.835 +/- 0.707 (mean +/- SD) pmol/hr of T-3 a nd 2.44 +/- 2.09 pmol/hr of T-4 per 100 g body weight (BW). Also, 8.19 to 11.2% of secreted T-4 was monodeiodinated to T-3, forming 0.200 to 0.274 pmol/hr of T-3 per 100 g BW. This means that 75 to 81% of all T -3 produced was secreted by the thyroid in these starved fish-a rather surprising result-while the remaining 19 to 25% resulted from T-4 to T-3 conversion. (C) 1996 Academic Press, Inc.