Hypothyroidism Increases Pancreatic Thyrotropin-Releasing Hormone Concentrations in Adult Rats

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Abstract
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 3 groups during 4, 6, or 9 wk by propylthiouracil (PUT) in drinking water and a low iodine diet (LID). Another group received PTU-LID for 6 wk, followed by 5 wk on a normal diet to restore euthyroidism. A possible toxic effect of PTU per se was eliminated by treating 1 control group with PTU and triiodothyronine. P-TRH and TRH-DA were measured by specific radioimmunoassay. In the hypothyroid groups, P-TRH concentrations (mean .+-. standard error of the mean) were increased 10-fold (6.95 .+-. 2.09; 5.51 .+-. 1.3; 9.79 .+-. 3.3 pg/(mg .cntdot. 100 g body wt), respectively, with a control value of 0.55 .+-. 0.39, P < 0.01). This increase was reversible, as shown by the group on PTU-LID followed by a normal diet (0.58 .+-. 0.39, not significant). P-TRH-DA present in the control group was decreased after 4 wk of PTU-LID treatment and totally abolished after 6 and 9 wk of PTU-LID treatment. Thyroid status apparently 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.