Kinetic analysis of thyroid hormone secretion and interconversion in the 5-day-fasted rainbow trout, Oncorhynchus mykiss.

Estimating the 3,5,3'-triiodothyronine (T3) thyroidal secretion rate and the rate of extrathyroidal conversion of thyroxine (T4) to T3 are two difficult and important quantitative endocrine system problems in vertebrates. To address these questions in fish, two thyroid hormone tracer studies were modeled and analyzed, based on data from two groups of rainbow trout maintained at 12 degrees and fasted for 5 days. The data consisted of three time series: plasma concentrations of radioactive 125I-labeled T3 (T3*) following T3* injection, and both labeled T4 (T4*) and T3* following T4* injection. To facilitate model parameter estimation, plasma volumes were determined independently by injection of labeled bovine serum albumin. The T4* injectate was contaminated by an unknown amount of T3*, and this was considered an additional unknown. A six-compartment model was formulated in terms of 13 uniquely identifiable (quantifiable) parameters, which were estimated simultaneously from the three data sets using a sophisticated optimization algorithm built into a new model-fitting software package called FITMOD. The rates of interest, plus other kinetic indices, were estimated successfully using additional analysis. We found that the thyroid gland secreted 0.835 +/- 0.707 (mean +/- SD) pmol/hr of T3 and 2.44 +/- 2.09 pmol/hr of T4 per 100 g body weight (BW). Also, 8.19 to 11.2% of secreted T4 was monodeiodinated to T3, forming 0.200 to 0.274 pmol/hr of T3 per 100 g BW. This means that 75 to 81% of all T3 produced was secreted by the thyroid in these starved fish--a rather surprising result--while the remaining 19 to 25% resulted from T4 to T3 conversion.