Altered diurnal rhythm of prolactin in systemic sclerosis

Objective. Mild hyperprolactinemia has been reported in systemic lupus eryt hematosus (SLE) and systemic sclerosis (SSc). We investigated whether the e levated serum level of prolactin (Prl) detected in SSc is due to a sustaine d increase over 24 h and/or a shift in the diurnal rhythm, and whether Prl autoantibodies - originally described in SLE - may interfere in the assay. Methods. The serum level of Prl was measured by ELISA and compared between 73 patients with SSc and 73 age and sex matched controls (78% women, age 56 +/- 11 years). The diurnal rhythms of Prl and thyrotropin (thyroid stimula ting hormone, TSH) were compared between 3 patients with SSc and 10 healthy controls. Blood was taken at 2-3, 6-7, 10-11 a.m., and 2-3, 6-7, 10-11 p.m . The serum level of Prl autoantibodies was measured by ELISA and compared between matched patients with SSc and SLE and controls (n = 42 each). Stand ard curves of the Prl ELISA were spiked with 10% sera containing high level s of Prl autoantibodies to test interference. Results. Serum levels of Prl measured in the morning (8-10 a.m.) were signi ficantly higher in patients with SSc (17.9 +/- 7.7 ng/ml), compared with co ntrols (9.3 +/- 4.2 ng/ml; p < 0.05). Ln SSc, 40% of patients had Prl level s > 20 ng/ml, but no correlation was found with Scl-70 or Prl autoantibodie s. Younger patients (< 50 years, n = 23/73) showed higher serum levels of P rl than older patients (21.3 +/- 10.3 vs 16.3 +/- 6.2 ng/ml; p < 0.05). The diurnal rhythm of Prl revealed that both a sustained increase over 24 h an d some shift occult-ed in SSc. Peaks of secretion were detected between 6 a nd 11 a.m., instead of 2-6 a.m. The median levels of TSH over 24 h in patie nts with SSc ranged within the normal limits. Nevertheless, in SSc, a signi ficant correlation (r = 0.59, p < 0.01) was found between diurnal rhythms o f Prl and TSH. The prevalence of Prl autoantibodies in serum was 8% in SSc, 27% in SLE, and < 5% in controls. However, the presence of Prl autoantibod ies did not interfere with our assay. Conclusion. Our data confirm that mild hyperprolactinemia occurs in a subgr oup of patients with SSc, and showed that the elevated serum level of Prl i s due to both a sustained increase over 24 h and a shift in the diurnal rhy thm. The correlation between diurnal rhythms of Prl and TSH suggests common regulatory mechanisms.