Growth hormone (GH) and prolactin share similarities in structure and function. We have previously shown that women with congenital isolated GH deficiency (IGHD) caused by a homozygous mutation in the GHRH receptor gene (GHRHR) (MUT/MUT) have a short reproductive life, with anticipated climacteric. At climacteric, they have lower prolactin levels than normal controls (N/N). Because they are able to breast feed, we hypothesized that this prolactin reduction is limited to climacteric, as result of lower estradiol exposure of the lactotrophs. The purposes of this work were to assess prolactin levels in broader age adults homozygous and heterozygous (MUT/N) for the mutation and in normal controls (N/N), and to correlate them to sex steroids levels. We enrolled 24 GH-naïve MUT/MUT (12 female), 25 MUT/N (14 female), and 25 N/N (11 female) subjects, aged 25-65 years. Anthropometric data and serum prolactin, estradiol, total testosterone, and sex hormone binding globulin (SHBG) were measured. Free testosterone was calculated. Prolactin levels were similar in the three groups. In males, testosterone and SHBG levels were higher in MUT/MUT in comparison to N/N. There was no difference in free testosterone among groups. In all 74 individuals, prolactin correlated inversely with age (p < 0.0001) and directly with serum estradiol (p = 0.018). Prolactin levels in subjects with IGHD due to a homozygous GHRHR mutation are similar to heterozygous and normal homozygous, but total testosterone and SHBG are higher in male MUT/MUT, with no difference in free testosterone. The reduced prolactin level is limited to climacteric period, possibly due to reduced estrogen exposure.