Besides the overproduction of cytokines such as IL-6 and TNF-α within the inflamed mammary microenvironment, which has deleterious effects on mammary gland function, obesity is linked to increased synthesis of serotonin in the mammary gland by upregulating tryptophan hydroxylase 1 (TPH1). Here, TPH1 is linked to obesity due to melanocortin 4 receptor deficiency.