Can LH and FSH Be Completely Flatlined in FHA?
Yes, LH and FSH can be completely flatlined (undetectable or <2 IU/L) in severe cases of Functional Hypothalamic Amenorrhea, though the more typical presentation is "low-normal" levels. 1
The Spectrum of Gonadotropin Suppression in FHA
The hormonal presentation in FHA exists on a spectrum of GnRH pulsatile suppression severity:
The hallmark finding is "low-normal" FSH and LH levels (typically in the 2-5 IU/L range), which represents the most common presentation and helps distinguish FHA from other causes of amenorrhea 2
However, severe cases can present with both LH and FSH <2 IU/L, which represents profound hypothalamic suppression and is sometimes referred to as hypothalamic hypogonadism (HH) 1
The spectrum of GnRH-LH disturbances in FHA is very broad and includes lower mean frequency of LH pulses, complete absence of LH pulsatility, normal-appearing secretion patterns, and even higher mean frequency of LH pulses in some cases 3
Clinical Context Matters
The degree of gonadotropin suppression correlates with severity of the underlying trigger:
Patients with more severe energy deficits, lower body weight, or more intense psychological stress tend to have more profound suppression of the hypothalamic-pituitary-gonadal axis 4
The fundamental pathophysiology involves reduced pulsatile secretion of GnRH from the hypothalamus, which leads to decreased LH pulse frequency and reduced FSH secretion 5
In some studies defining FHA with strict criteria, patients had to have both LH and FSH <2 IU/L to distinguish them from other conditions, demonstrating that this degree of suppression is recognized in the literature 1
Diagnostic Implications
When you encounter completely flatlined gonadotropins:
LH and FSH both <2 IU/L with estradiol <100 pmol/L strongly indicates hypothalamic hypogonadism/severe FHA rather than PCOS or other conditions 1
This pattern requires careful evaluation to exclude other causes of hypogonadotropic hypogonadism including Kallmann syndrome, normosmic idiopathic hypogonadotropic hypogonadism, drug-induced suppression (opiates, anabolic steroids), or pituitary pathology 5
The clinical context is critical: a clear history of energy deficit, caloric restriction, excessive exercise, or psychological stress in a young woman with amenorrhea supports FHA even with undetectable gonadotropins 2
Common Pitfall to Avoid
Do not assume that "low-normal" means gonadotropins cannot be lower - the term "low-normal" describes the typical presentation, but severe FHA can present with profoundly suppressed or undetectable levels 1, 2
Approximately 29% of FHA patients have serum basal FSH <4.5 IU/L (below the 5th percentile of normal values), and some will have even lower levels approaching undetectable ranges 6
The key distinguishing feature from primary ovarian insufficiency is that POI shows FSH >40 IU/L, the complete opposite of FHA 7