Obtaining Testosterone and FSH levels are minimum baseline hormones needed to assess fertility status affording the ability to differentiate between subtypes of subfertility or infertility.
The hypothalamic-pituitary-gonadal (HPG) axis drives male sexual development and fertility. GnRH is secreted by the hypothalamus in a pulsatile manner. It is the distinct frequency and amplitude of these pulses that directly drive LH and FSH secretion from the anterior pituitary.
References Assessing Testicular Reserve.pdf