Comparison of the effects of the L-dopa and insulin tolerance tests on cortisol secretion

The aims of the present study are to evaluate the effect of L-dopa on the secretion of cortisol and adrenocorticotropic hormone (ACTH) in short children and compare the performance of this test with the insulin tolerance test (ITT) in a large number of patients.

A total of 29 short but otherwise healthy children [mean age 9.5 ± 3.1 years (range 3.7-14.9 years)] who had inadequate growth hormone (GH) responses to ITT, which was performed as the first test, were consecutively enrolled in this study. GH, cortisol, and ACTH levels were measured just before administration of L-dopa and then at 30-min intervals afterward over a total time of 120 min. Peak concentrations of cortisol and ACTH exceeding 18 µg/dL (496 mmol/L) and 46 pg/mL (10.2 pmol/L), respectively, were defined as an adequate response.

While the L-dopa test revealed that 26 of the 29 children (89.7%) had peak serum cortisol levels of > 18 µg/dL, the ITT revealed that only 23 children (79.3%) had adequate cortisol responses. The L-dopa test revealed normal ACTH responses (> 46 pg/mL) in 24 (82.8%) patients. Peak cortisol levels were higher in children with normal ACTH responses than in those with subnormal ACTH responses (25.6 ± 6.2 vs. 19.5 ± 6.4 µg/dL, p = 0.054), but the difference observed was statistically insignificant.

The results of the current study confirm that the L-dopa test is a reliable test of cortisol secretion. As such, this test may be applicable to assessments of the hypothalamic-pituitary-adrenal axis.