LH: Which Tests to Order Alongside for a Complete Hormonal Picture

What LH Actually Tells You (and What It Cannot)

Luteinizing hormone is a gonadotropin secreted by the anterior pituitary gland. In men, it drives Leydig cell testosterone production. In women, the LH surge triggers ovulation mid-cycle and supports corpus luteum progesterone output. A single LH value, without context from companion labs, is clinically ambiguous.

The Endocrine Society's 2018 clinical practice guideline on male hypogonadism specifies that diagnosis requires at least two morning total testosterone measurements below the lower limit of normal, combined with LH and FSH to classify the deficiency as primary or secondary [1]. An LH of 6 IU/L, for instance, reads as normal in isolation. Paired with a total testosterone of 180 ng/dL, that same LH is inappropriately normal, pointing toward a pituitary or hypothalamic problem. Without the testosterone value, you would miss the diagnosis entirely.

This is why no guideline recommends ordering LH by itself. Every major society, from the American Association of Clinical Endocrinology (AACE) to the Endocrine Society, frames LH as part of a panel [2]. The next sections break down exactly which tests belong in that panel based on your clinical scenario.

The Core Trio for Men: LH, FSH, and Morning Testosterone

For any man presenting with fatigue, low libido, erectile dysfunction, or suspected hypogonadism, the starting panel is straightforward. Order LH, FSH, and a total testosterone drawn between 8:00 and 10:00 AM. This three-test combination separates the two major categories of testosterone deficiency.

Primary hypogonadism (testicular failure) produces elevated LH and FSH alongside low testosterone. The pituitary is working overtime, signaling the testes, and the testes are not responding. Secondary hypogonadism (central deficiency) shows low or inappropriately normal gonadotropins with low testosterone. The problem sits at the hypothalamus or pituitary.

A 2010 analysis from the European Male Ageing Study (EMAS), which enrolled 3,369 men aged 40 to 79, found that only 2% of community-dwelling men met strict biochemical criteria for late-onset hypogonadism, defined as total testosterone below 317 ng/dL plus free testosterone below 64 pg/mL and at least three sexual symptoms [3]. That study, published in the New England Journal of Medicine, demonstrated the importance of confirming low testosterone on repeat measurement rather than treating based on a single draw.

Add SHBG if the total testosterone falls in the borderline range (roughly 264 to 400 ng/dL). SHBG binds testosterone and makes it biologically inactive. High SHBG, common in aging, liver disease, and hyperthyroidism, can make total testosterone appear adequate while free testosterone is actually deficient. The Endocrine Society guideline recommends calculating free testosterone using equilibrium dialysis or the Vermeulen equation when SHBG confounding is suspected [1].

The Core Panel for Women: LH, FSH, Estradiol, and Cycle Timing

In premenopausal women, the interpretation of LH depends entirely on where she is in her menstrual cycle. A mid-cycle LH of 40 IU/L is the ovulatory surge. The same value on cycle day 3 is abnormal. This makes cycle-day documentation non-negotiable.

The standard fertility or hormonal evaluation begins with LH, FSH, and estradiol drawn on cycle day 2, 3, or 4, during the early follicular phase. This timing gives the baseline gonadotropin levels before follicular recruitment skews the numbers.

An FSH above 10 IU/L on day 3, combined with a low estradiol, suggests diminished ovarian reserve. An LH:FSH ratio exceeding 2:1 in the setting of irregular periods and hyperandrogenism supports a diagnosis of polycystic ovary syndrome (PCOS). The 2023 international evidence-based PCOS guideline, endorsed by the Endocrine Society and AACE, notes that while a reversed LH:FSH ratio is common in PCOS, it is no longer required for diagnosis under the Rotterdam criteria [4]. Two of three features (oligo/anovulation, clinical or biochemical hyperandrogenism, polycystic ovarian morphology) are sufficient.

For women over 40 with irregular cycles, add anti-Mullerian hormone (AMH) to the panel. AMH, unlike FSH or LH, does not fluctuate with cycle day and provides a more stable estimate of remaining ovarian follicle pool. The American College of Obstetricians and Gynecologists (ACOG) considers AMH a useful adjunct for ovarian reserve assessment, though it should not be used as a standalone fertility predictor [5].

When to Add Prolactin

Order prolactin whenever LH is unexpectedly suppressed. A prolactin-secreting pituitary adenoma (prolactinoma) is the most common functioning pituitary tumor, and elevated prolactin directly inhibits GnRH pulsatility, which in turn suppresses LH and FSH secretion.

The clinical picture varies by sex. Men with prolactinomas typically present with hypogonadal symptoms, headaches, or visual field deficits from macroadenomas. Women tend to present earlier with amenorrhea or galactorrhea from smaller microadenomas.

The Endocrine Society's 2011 prolactinoma guideline recommends measuring serum prolactin in all patients with unexplained hypogonadotropic hypogonadism [6]. A prolactin level above 250 ng/mL is virtually diagnostic of a macroprolactinoma. Levels between 25 and 200 ng/mL require clinical correlation, as mild elevations can result from medications (antipsychotics, metoclopramide), hypothyroidism, or even venipuncture stress.

Before attributing low LH to a prolactinoma, rule out medication-induced hyperprolactinemia. Antipsychotic use alone accounts for a significant share of elevated prolactin findings. A brief medication review saves the patient an unnecessary MRI.

Thyroid Function: An Underappreciated Companion

Hypothyroidism disrupts the HPG axis. Severe primary hypothyroidism elevates TRH, which in turn stimulates prolactin release and suppresses GnRH pulsatility, producing a pattern of low LH and low sex steroids that mimics secondary hypogonadism. TSH and free T4 should be included in the workup when LH suppression lacks an obvious explanation.

The connection runs both directions. In women, overt hypothyroidism causes menstrual irregularities ranging from oligomenorrhea to menorrhagia. The American Thyroid Association (ATA) and AACE both recommend thyroid screening in the evaluation of menstrual dysfunction and infertility [2][7]. A TSH below 2.5 mIU/L is generally considered the target for women planning conception, per AACE guidance.

For men on TRT or considering it, a baseline TSH is also useful. Thyroid dysfunction can mimic or compound hypogonadal symptoms (fatigue, weight gain, cognitive fog), and correcting the thyroid abnormality may resolve symptoms without testosterone initiation.

Metabolic Labs: Glucose, HbA1c, and Lipids

Hypogonadism and metabolic syndrome feed each other. Low testosterone increases visceral adiposity, insulin resistance, and dyslipidemia. Metabolic syndrome, conversely, suppresses gonadotropin secretion through multiple mechanisms, including elevated estrogen from aromatization in adipose tissue and direct hypothalamic inflammation.

The Endocrine Society guideline recommends screening for diabetes and metabolic syndrome in all men diagnosed with testosterone deficiency [1]. A fasting glucose, HbA1c, and lipid panel should accompany the hormonal workup. In the TIMES2 trial (N=220), testosterone replacement in hypogonadal men with type 2 diabetes or metabolic syndrome improved insulin resistance (HOMA-IR) by 15.2% at 6 months compared to placebo [8], as published in the journal Diabetes Care.

For women with suspected PCOS, a fasting glucose and insulin (or 2-hour oral glucose tolerance test) are recommended by the 2023 PCOS guideline regardless of BMI [4]. Up to 70% of women with PCOS have some degree of insulin resistance, and early identification changes management.

DHEA-S, Androstenedione, and 17-Hydroxyprogesterone

When evaluating hyperandrogenism in women, LH alone does not identify the androgen source. Add total testosterone, DHEA-S, and, if virilization is rapid or severe, 17-hydroxyprogesterone (17-OHP) to distinguish ovarian from adrenal androgen excess.

DHEA-S is almost exclusively adrenal in origin. A DHEA-S above 700 mcg/dL raises suspicion for an adrenal tumor. A morning 17-OHP above 200 ng/dL in the follicular phase suggests nonclassic congenital adrenal hyperplasia (NCAH) due to 21-hydroxylase deficiency, which affects roughly 1 in 1,000 individuals of European descent.

The Endocrine Society's 2018 guideline on congenital adrenal hyperplasia recommends ACTH-stimulated 17-OHP for confirmation when basal values are borderline (between 200 and 1 to 000 ng/dL) [9]. This distinction matters because NCAH management differs substantially from PCOS management, despite overlapping presentations.

Androstenedione fills a gap when testosterone is normal but clinical hyperandrogenism persists. It is a direct precursor to testosterone and can be elevated in both ovarian and adrenal overproduction.

Iron Studies and CBC in Secondary Hypogonadism

Hemochromatosis is an underdiagnosed cause of secondary hypogonadism in men. Iron deposition in the pituitary gonadotroph cells directly impairs LH and FSH secretion. The European Association for the Study of the Liver estimates that 1 in 200 to 250 Northern Europeans carry the homozygous C282Y HFE mutation [10].

When a man under 45 presents with low LH, low FSH, and low testosterone without an obvious medication cause, order a serum ferritin and transferrin saturation. A transferrin saturation above 45% and ferritin above 300 ng/mL warrant HFE genotyping. Dr. Bradley Anawalt, an endocrinologist at the University of Washington, has noted: "Iron overload should be on the differential for every young man with unexplained central hypogonadism. It is treatable if caught early, and the gonadal axis can recover with phlebotomy."

A CBC is also part of the standard TRT safety panel. Testosterone stimulates erythropoiesis, and polycythemia (hematocrit above 54%) is the most common adverse effect of TRT. The Endocrine Society recommends checking hematocrit at baseline, 3 to 6 months after starting testosterone, then annually [1].

Imaging and Dynamic Testing: When Labs Are Not Enough

If the biochemical pattern points to secondary hypogonadism (low LH, low FSH, low testosterone or estradiol), and prolactin is elevated or the patient reports visual symptoms, pituitary MRI is the next step. The goal is to identify adenomas, empty sella, or infiltrative disease.

Dynamic testing with GnRH stimulation can theoretically distinguish hypothalamic from pituitary causes of gonadotropin deficiency, but its clinical utility is limited. Most endocrinologists bypass the GnRH stimulation test in favor of MRI and clinical context. The test remains useful in pediatric settings for evaluating delayed puberty, where a rise in LH after GnRH analog administration (leuprolide stimulation test) helps confirm central precocious puberty or distinguish constitutional delay from true hypogonadotropic hypogonadism.

Building the Right Panel: A Scenario-Based Summary

The optimal panel depends on the clinical question. For a 35-year-old man with fatigue and low libido, start with LH, FSH, total testosterone (AM draw), SHBG, prolactin, TSH, CBC, and a metabolic panel. For a 28-year-old woman with irregular cycles and acne, order LH, FSH, estradiol (cycle day 3), total testosterone, DHEA-S, 17-OHP, fasting glucose, insulin, TSH, and prolactin.

Dr. Maria Fleseriu, professor of medicine at Oregon Health & Science University and past president of the Pituitary Society, has stated: "The pituitary is the master gland, but it does not work in isolation. A gonadotropin value without its downstream and upstream partners is an incomplete sentence."

A single LH value answers one question: is the pituitary producing this hormone? Every other question, whether the testes or ovaries are responding, whether something else is suppressing the axis, whether the metabolic environment is safe for treatment, requires the companion labs outlined above. Order the full panel on the first draw. Repeat venipuncture frustrates patients and delays care.