Combined pituitary stimulation test: interactions of hypothalamic releasing hormones in man

The use of hypothalamic releasing hormones for the clinical assessment of anterior pituitary function is both simple and free of severe side effects. Tests with the recently discovered substances GRF and CRF as well as with combinations of several releasing hormones are therefore used in many clinics. A reliable interpretation of such combined tests, however, is only possible when positive or negative interactions between these releasing hormones are known. After a rest of 2 h to reach basal Cortisol levels, 7 groups of 5 male volunteers each received an iv bolus injection consisting of either: A): GRF (100 µg) + CRF (50 µg) + TRH (200 jug) + LHRH (100 µg), B): CRF + TRH, C): GRF + TRH, D): LHRH + TRH, E): TRH, F): GRF, G): CRF. During the following 2 h, GH, TSH, Cortisol, LH, FSH and prolactin were measured every 15 min. The TSH response after the injection of all 4 releasing hormones was significantly higher (Δ TSH = 16.5 ± 2.0 µU/ml, x ± SE) compared to the injection of TRH alone (Δ TSH = 9.3 ± 1.4 µU/ml; p< 0.025). This increment in TSH secretion was confirmed when 2 groups of 5 female volunteers were studied with the TRH-test (Δ TSH = 9.9 ± 1.8 µU/ml) or the combination of all four releasing hormones (Δ TSH = 16.8 ± 2.9 µU/ml; p < 0.05). Thisexaggerated TSH-response to TRH was demonstrated to be entirely due to simultaneous administration of GRF, whereas CRF and LHRH in combination with TRH had no additional effecton TSH release. For all other hormones measured no significant differences could be found between single and combined tests.