Aspects of chronic oral treatment with thyrotropin-releasing hormone: the hypothalamic-pituitary-thyroid axis in rats. A study with a pharmacological dose of thyrotropin-releasing hormone.

The effects of 16 days of oral treatment with thyrotropin-releasing hormone (TRH, 1 mg/24 h) on serum levels of thyrotropin (TSH), thyroxine (T4) and triiodothyronine (T3) and the kinetics of TRH in the blood were studied in normal rats. A second group of animals served as controls. TRH was dissolved by sonification (10 mg/l) and was stable in tap water. TRH was measured by a radioimmunoassay procedure (normal range: 20-80 pmol/l, antiserum K2B9 1:120,000 final dilution). An increase in basal TSH (7,200 +/- 440 ng/l, mean +/- SD) was found after 2 days of treatment (11,420 +/- 810 ng/l), but a significant increase was observed after 5 days of treatment (12,530 +/- 640 ng/l, p less than 0.001). T4 serum concentrations remained in the normal range during the entire period of study, whereas T3 serum concentrations (0.76 +/- 0.1 micrograms/l) were increased to 1.22 +/- 0.2 micrograms/l on day 5 (p less than 0.001). A subsequent decline of TSH, T4 and T3 up to the end of the study was observed. TRHmax concentrations were registered on day 5 (790 +/- 24 pmol/l). The mean value of TRHmax was 723 +/- 34 pmol/l. To improve the stability of TRH in tap water, 1-ml samples of drinking water with dissolved TRH were measured. The mean TRH concentration in drinking water was 73 +/- 1.5% (SD). No significant correlations were found between the area under the curve of TSH (184,340 ng.l-1.24 h) and that of TRH (14,954 pmol.l-1.24 h).(ABSTRACT TRUNCATED AT 250 WORDS)