Isolated growth hormone deficiency type II (IGHD II) is characterized by short stature due to dominant-negative mutations of the human growth hormone gene (GH1). Most of the known mutations responsible for IGHD II cause aberrant splicing of GH1 transcripts. We have recently shown that mutations that cause exon 3 skipping and produce a dominant-negative 17.5-kDa isoform in humans also cause a dose-dependent disruption of GH secretory vesicles when expressed in GC cells and transgenic mice. We show here that overexpression of the dominant-negative 17.5-kDa isoform also destroys the majority of somatotrophs, leading to anterior pituitary hypoplasia in transgenic mice. It is, therefore, important to understand the regulation of GH1 splicing and why its perturbation causes IGHD II. We demonstrate that dual splicing enhancers are required to ensure exon 3 definition to produce full-length 22-kDa hormone. We also show that splicing enhancer mutations that weaken exon 3 recognition produce variable amounts of the 17.5-kDa isoform, a result which could potentially explain the clinical variability observed in IGHD II. Non-canonical splicing mutations that disrupt splicing enhancers, such as those illustrated here, demonstrate the importance of enhancer elements in regulating alternative splicing to prevent human disease.