LH
As everyone knows, it is LH which stimulates the Leydig cells of the testes to produce testosterone. A caveat, however: LH has a half-life of only about 30 minutes. When you combine this fact with the absolute pulsatile nature of its pituitary release, care must be taken to not place too much weight upon a single draw. A luxury would be to acquire serial draws, say, twenty minutes apart. However, such would be both inconvenient and probably prohibitively expensive for the patient. The most important reason to assay the gonadotrophins is to differentiate between primary and secondary (hypogonadotrophic) hypogonadism.
FSH
The eight hour half-life of this hormone makes it a better marker for gonadotrophin production. It is also less an acute phase reactant to varying serum androgen and estrogen levels than LH. Greatly elevated FSH levels could signal a gonadotrophin-secreting pituitary tumor.
Of note, I run FSH (but not LH) on the follow-up labs, the new third generation (“sensitive”) assay, to determine the magnitude of HPTA suppression secondary to androgen therapy. It also provides valuable information for those patients undergoing TRT who are interested in the state of their fertility.
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