Hypothyroidism increases pancreatic thyrotropin-releasing hormone concentrations in adult rats

The effect of hypothyroidism on pancreatic TRH (P-TRH) and P-TRH-degrading activity (P-TRH-DA) was studied in adult rats. Hypothyroidism was induced in three groups during 4, 6, or 9 weeks by propylthiouracil (PTU) in drinking water and a low iodine diet (LID). Another group received PTU-LID for 6 weeks, followed by 5 weeks on a normal diet to restore euthyroidism. A possible toxic effect of PTU per se was eliminated by treating one control group with PTU and T3. P-TRH and TRH-DA were measured by specific RIA. In the hypothyroid groups, P-TRH concentrations (mean +/- SEM) were increased 10-fold (6.95 +/- 2.09; 5.51 +/- 1.3; 9.79 +/- 3.3 pg/(mg X 100 g BW), respectively, with a control value of 0.55 +/- 0.39, P less than 0.01). This increase was reversible, as shown by the group on PTU-LID followed by a normal diet (0.58 +/- 0.39, NS). P-TRH-DA present in the control group was decreased after 4 weeks of PTU-LID treatment and totally abolished after 6 and 9 weeks of PTU-LID treatment. In conclusion, these results suggest that thyroid status modulates P-TRH concentrations. This effect may be due to the disappearance of the TRH-DA in response to hypothyroidism. P-TRH stores may be regulated by the enzyme(s) involved in P-TRH-DA.