Leptin is primarily expressed in white adipose tissue; however, it is expressed in the hypothalamus and reproductive tissues as well. Leptin acts by activating the leptin receptors Ob-Rs. Additionally, the regulation of several neuroendocrine and reproductive functions, including the inhibition of glucocorticoids and enhancement of thyroxine and sex hormone concentrations in human beings and mice are leptin functions.
It has been suggested that thyroid hormones TH could directly regulate leptin expression. Additionally, hypothyroidism compromises the intracellular integration of leptin signaling specifically in the arcuate nucleus. The effects of TH involve the proliferation and differentiation of Sertoli and Leydig cells during development, spermatogenesis, and steroidogenesis. In this context, TH disorders are associated with sexual dysfunction. Further studies are necessary to clarify the effects of both hormones in the testis during hypothyroidism.
The goal of this review is to highlight the current knowledge regarding leptin and TH in the testis. The role of thyroid hormones TH and leptin in testicular physiology is not fully understood.
Receptors for both hormones are present in the testis 1 , 2 , and some reproductive functions have been described as being a result of TH or leptin actions in the testis, although the interaction between these two hormones should be further investigated. Leptin plays a key role in body weight homeostasis and has recently emerged as a relevant neuroendocrine mediator in different cell types, including testicular cells 3. This hormone appears to act by inhibiting testicular steroidogenesis, leading to reduced levels of testosterone and modulation of gene expression Figure 1.
Figure 1. The major pathways by which leptin and TH interact with the hypothalamicโpituitary axis to regulate testis function. The hypothalamicโpituitaryโgonadal axis controls reproduction. GnRH is secreted from the hypothalamus by GnRH expressing neurons, stimulating the anterior portion of the pituitary gland to produce and secrete LH and FSH, whose function is to stimulate the gonads to produce estrogen and testosterone.
