Autoradiographic thyroid evaluation in short-term experimental diabetes mellitus

Previous studies have shown that in vitro thyroid peroxidase (TPO) iodide oxidation activity is decreased and thyroid T4-5'-deiodinase activity is increased 15 days after induction of experimental diabetes mellitus (DM). In the present study we used thyroid histoautoradiography, an indirect assay of in vivo TPO activity, to determine the possible parallelism between the in vitro and in vivo changes induced by experimental DM. DM was induced in male Wistar rats (about 250 g body weight) by a single i.p. streptozotocin injection (45 mg/kg), while control (C) animals received a single injection of the vehicle. Seven and 30 days after diabetes induction, each diabetic and control animal was given i.p. a tracer dose of 125I (2 microCi), 2.5 h before thyroid excision. The glands were counted, weighted, fixed in Bouin's solution, embedded in paraffin and cut. The sections were stained with HE and exposed to NTB-2 emulsion (Kodak). The autohistograms were developed and the quantitative distribution of silver grains was evaluated with a computerized image analyzer system. Thyroid radioiodine uptake was significantly decreased only after 30 days of DM (C: 0.38 +/- 0.05 vs DM: 0.20 +/- 0.04%/mg thyroid, P < 0.05) while in vivo TPO activity was significantly decreased 7 and 30 days after DM induction (C: 5.3 and 4.5 grains/100 micron 2 vs DM: 2.9 and 1.6 grains/100 micron 2, respectively, P < 0.05). These data suggest that insulin deficiency first reduces in vivo TPO activity during short-term experimental diabetes mellitus.