When to Measure LH and FSH
Measure LH and FSH as part of the initial endocrine evaluation in any patient presenting with amenorrhea, irregular menstrual cycles, suspected hypogonadism, or infertility after ruling out pregnancy. 1, 2
Primary Clinical Indications
In Women with Amenorrhea or Irregular Cycles
Obtain LH and FSH levels immediately after confirming negative pregnancy test in women presenting with amenorrhea or irregular menstrual cycles. 1 The hormone panel should include:
- Serum LH
- Serum FSH
- Estradiol (E2)
- TSH and free T4
- Prolactin
- Anti-Müllerian hormone (AMH)
Timing consideration: These can be measured at any point during amenorrhea, as there is no menstrual cycle to time around. The pattern of results matters more than absolute values. 1
In Men with Suspected Testosterone Deficiency
Measure serum LH in all men with confirmed low testosterone levels. 2 This is a strong recommendation that helps establish whether the testosterone deficiency is:
- Primary hypogonadism (elevated LH with low testosterone)
- Secondary hypogonadism (low or low-normal LH with low testosterone)
Additional testing algorithm for men:
- If LH is low or low-normal with low testosterone → measure serum prolactin 2
- If prolactin is elevated → repeat to confirm, then refer to endocrinology 2
- If total testosterone <150 ng/dL with low/normal LH → obtain pituitary MRI regardless of prolactin level 2
High-Risk Populations Requiring Screening
Measure LH and FSH even without symptoms in patients with: 2
- Unexplained anemia
- Bone density loss
- Diabetes
- History of chemotherapy exposure
- Direct or scatter radiation to testes/pelvis
- HIV infection
- Chronic narcotic use
- Known pituitary disorders
- Chronic corticosteroid use
- Infertility
Interpretation Framework
Pattern Recognition Over Absolute Values
The relationship between LH, FSH, and sex steroids provides diagnostic clarity: 1
Ovarian insufficiency pattern:
- Elevated FSH and LH
- Low E2 (<20 pg/mL)
- Low progesterone (<1 ng/mL)
Functional hypothalamic amenorrhea pattern:
- Low or low-normal LH
- Normal FSH (usually higher than LH)
- Low E2 (<50 pg/mL)
- Preserved GnRH response (2-3 fold rise with stimulation)
Midcycle surge pattern:
- High FSH and LH
- E2 >150 pg/mL
- Progesterone <2 ng/mL
Organic hypothalamic/pituitary disease:
- Very low or undetectable LH and FSH
- Low sex steroids
- Consider imaging
Common Pitfall: Normal Gonadotropins Don't Exclude Pathology
A confusing clinical scenario: In functional hypothalamic amenorrhea, LH and FSH are often in the "normal" range despite low estradiol. 1 This misleads clinicians who expect low gonadotropins. The key is recognizing that these "normal" levels are inappropriately normal for the degree of hypoestrogenism—they should be elevated if ovarian function were the problem.
Special Considerations
Timing in Cycling Women
For women with irregular but present cycles, measure during early follicular phase (days 2-5) when evaluating baseline ovarian reserve. 3, 4 FSH levels progressively increase with reproductive aging starting as early as age 29-30 years, with LH increases becoming significant around age 35-36. 4
Before Fertility Treatment
Men interested in fertility must have reproductive evaluation including LH and FSH before starting testosterone therapy, as testosterone suppresses gonadotropins and spermatogenesis. 2 This is critical because testosterone therapy will confound future interpretation and impair fertility.
Avoid Screening Questionnaires
Do not use screening questionnaires as substitutes for laboratory LH and FSH measurement. 2 Their sensitivities and specificities vary too widely to be clinically reliable for diagnosis or treatment decisions.