RED-S Blood Work and The Female Athlete Triad: A Screening Guide for Coaches and Clinicians

Introduction

Some of my patients are female athletes who naturally train at a high level. In many sports, there are also strict weight categories and competition diets that must be followed. From time to time, a patient comes to my clinic who has lost her menstrual cycle, is experiencing recurrent injuries, or notices a decline in performance. These same patients may also suffer from depression, burnout, and persistent fatigue. However, their blood tests may still appear normal — which can pose a significant challenge for the inexperienced clinician.

The Female Athlete Triad — originally defined as the interrelation of low energy availability, menstrual dysfunction, and low bone mineral density — has since been expanded into the broader framework of Relative Energy Deficiency in Sport (RED-S), which recognises that low energy availability disrupts virtually every physiological system, not just the reproductive and skeletal axes. The 2023 International Olympic Committee consensus statement on RED-S describes this syndrome as strongly associated with deleterious health and performance outcomes across the endocrine, metabolic, skeletal, cardiovascular, and immune systems [1].

I wrote this article to provide a practical, evidence-based blood work screening guide for coaches, athletic trainers, and clinicians working with female athletes. It covers the key hormonal biomarkers, indirect indicators of energy availability, menstrual dysfunction markers, and bone health indices — along with clear guidance on when clinical escalation is warranted. This article provides a practical, evidence-based RED-S blood work screening guide for coaches, athletic trainers, and clinicians working with female athletes. For a broader overview of which tests are most valuable in athletic populations, see Which Blood Tests Do Athletes Actually Need?

RED-S Blood Work and Low Energy Availability

Before examining individual biomarkers, it is essential to understand the mechanism connecting them. Energy availability is defined as dietary energy intake minus exercise energy expenditure, expressed relative to fat-free mass. Research demonstrates that luteinizing hormone (LH) pulsatility disruption is commonly observed around or below 30 kcal per kilogram of fat-free mass per day, representing a widely used clinical risk threshold — though later work emphasises that individual responses vary and this should not be treated as a universal breakpoint [2]. Below this range, pulsatile LH secretion — the driver of ovarian function — becomes suppressed, triggering a chain of downstream endocrine consequences that no single laboratory value will fully capture [2].

A 2024 systematic review of 13 studies identified 79 markers used in diagnosing RED-S, categorized across six groups: bone mineral density, resting metabolic rate, blood biomarkers, anthropometrics, nutritional intake, and performance parameters. Of these, the most frequently utilized blood biomarker was triiodothyronine (T3) concentration, reflecting its sensitivity to energy status[3]. The key message for clinicians: no single value rules RED-S in or out. Blood work is a multi-marker clinical picture, not a one-item checklist. This same principle applies to athletic blood work interpretation broadly — as explored in Endurance Athlete Blood Ranges: Why Standard Lab Values Miss Performance-Relevant Abnormalities.

Athletic patients — particularly female athletes — are often accustomed to functioning while fatigued, training, working, and studying despite persistent tiredness. In many cases, they have developed years of discipline and may not even recognize what it feels like to be well-rested. For some female patients, the most obvious clinical sign is the loss of the menstrual cycle. However, many use oral contraceptives, which induce withdrawal bleeding rather than reflecting endogenous menstrual function. As a result, it may only become apparent after discontinuing the pill that normal menstrual function does not return.

Reproductive Hormones in RED-S Blood Work

Estradiol, LH, and FSH

The hypothalamic-pituitary-ovarian (HPO) axis is exquisitely sensitive to energy status. When Low Energy Availability (LEA) becomes problematic, the hypothalamus reduces gonadotropin-releasing hormone (GnRH) pulsatility, which in turn suppresses LH and FSH output from the anterior pituitary, and consequently reduces ovarian estradiol and progesterone production [4].

Estradiol (E2): Chronically markedly low estradiol in a premenopausal athlete is a red flag for functional hypothalamic amenorrhea (FHA). Research confirms that long-term LEA produces significant decreases in estradiol and progesterone in naturally menstruating athletes [4]. What constitutes a clinically concerning level is assay-dependent and must be interpreted in context — any result substantially below the expected follicular-phase range in a non-contraceptive user warrants further clinical evaluation.

LH and FSH: In FHA secondary to LEA, LH pulsatility is typically suppressed while basal LH concentrations may be low, normal, or even variable — making single-point measurements unreliable in isolation[5]. A pattern of low or low-normal LH with normal or low FSH, combined with low estradiol and absent or irregular menses, is clinically suggestive of hypothalamic suppression. Athletes using combined hormonal contraceptives can suppress endogenous HPO-axis markers and may mask energy-status-related reproductive dysfunction, requiring an alternative approach to monitoring[5].

In most cases, a careful clinical history already provides strong guidance. If a patient presents with clearly high training volume, low energy intake, and loss of the menstrual cycle, the first step is often to prioritize rest and increase caloric intake.

In practice, however, this is psychologically very challenging for many patients. Their identity may be closely tied to their athletic career, and reducing training is not something they readily accept. Similarly, some patients with anorexia may engage in compulsive exercise, mimicking a similar clinical picture. These cases are often particularly difficult to manage and typically require integrated psychiatric care as part of treatment.

If the patient is able to increase energy availability and reduce training load but menstrual function still does not return, further evaluation is warranted. In such cases, first-line hormonal testing should include estradiol (E2), LH, and FSH, and consultation with a gynecologist should be considered early. For context on how the broader hormonal picture connects to blood work patterns, see Female Athlete Bloodwork: A Complete Guide to Key Markers, Optimal Ranges, and Performance Implications.

Indirect Blood Markers of Energy Availability

Because direct measurement of energy availability in free-living athletes is methodologically difficult, blood biomarkers provide clinically actionable indirect evidence of chronic LEA. Research suggests that free and total sex hormones, gonadotropins, and T3 are among the most sensitive markers of early hypothalamic downregulation. [6].

However, it is important to remember that clinical examination and a thorough patient history often provide far more meaningful insight than a collection of laboratory tests.

Triiodothyronine (T3)

Free T3 is among the earliest and most reliable hormonal indicators of LEA. Controlled caloric restriction trials consistently demonstrate a 10–25% decrease in circulating T3, while T4 concentrations tend to remain stable in the short term[6]. Cross-sectional data show that athletes with functional hypothalamic amenorrhea tend to have lower free T3 levels compared to eumenorrheic athletes[3].

The physiological rationale is straightforward: the body downregulates peripheral conversion of T4 to T3 under energy deficit to reduce metabolic expenditure — an adaptive survival mechanism that becomes pathological when chronic. In structured athlete screening frameworks, a free T3 below 3.5 pmol/L has been used as a secondary indicator of LEA [12] — though this should be treated as a context- and laboratory-dependent screening clue rather than a universal diagnostic boundary, and always interpreted alongside the full clinical picture.

One critical clinical point: TSH typically remains normal in LEA-related T3 suppression because the disruption occurs at the peripheral conversion level, not at the pituitary. Ordering TSH alone — the default in most standard panels — completely misses this picture. This distinction is explored in depth in Thyroid Function in Athletes: Why TSH Alone Isn’t Enough.

Low T3 — or even globally reduced thyroid hormone levels — does not in itself establish the diagnosis. The diagnosis is based on the overall clinical picture, with thyroid values serving as supportive findings rather than definitive markers.

In practice, thyroid function is often assessed primarily to exclude primary hypothyroidism. The relative suppression of thyroid hormones seen in athletes is typically identified as a secondary or incidental finding within this broader clinical context.

Insulin-Like Growth Factor-1 (IGF-1)

IGF-1 is markedly suppressed during LEA, with reductions observed across multiple controlled studies in both short-term and long-term energy restriction[4]. Its relevance extends beyond energy monitoring: IGF-1 plays a direct role in maintaining bone formation by stimulating osteoblast activity, and reductions in IGF-1 may contribute to impaired bone remodeling in athletes with low energy availability[8]. An IGF-1 in the lower quartile of the age-adjusted reference range in an endurance or aesthetic sport athlete should prompt clinical context review.

Its relevance extends beyond energy monitoring: IGF-1 plays a direct role in maintaining bone formation by stimulating osteoblast activity, and reductions in IGF-1 may contribute to impaired bone remodeling in athletes with low energy availabilit

In clinical practice, IGF-1 is rarely measured outside of specific endocrinological indications. It is not part of routine laboratory panels and therefore has limited practical relevance in everyday diagnostic work. In this context, IGF-1 is better understood as a research-oriented marker rather than a standard tool in routine clinical assessment.

Cortisol

Elevated cortisol reflects activation of the hypothalamic-pituitary-adrenal axis under energy stress and is often observed in states of LEA[4]. However, cortisol is highly variable due to circadian rhythm and acute exercise effects. A single measurement has limited diagnostic specificity for RED-S and is most useful when chronically elevated values accompany other LEA indicators across the broader panel. For a deeper look at how cortisol interacts with training load and overtraining, see Cortisol and Overtraining: Understanding the Hidden Stress-Response Failure andT:C Ratio in Overtraining: Understanding the Testosterone-to-Cortisol Balance.

In my clinical practice, cortisol is of limited practical use in this context. To interpret it meaningfully, one would need to know the individual’s baseline level, ideally measured before the onset of symptoms. In addition, cortisol tends to return to baseline over time, which further limits its value as a diagnostic marker. In practice, cortisol is measured mainly in specific endocrinological situations. It is not particularly useful for identifying low energy availability and is, in my view, largely a research-oriented marker rather than a tool for routine clinical assessment

Leptin and Fasting Insulin

Leptin consistently decreases in response to LEA and low body fat, reflecting reduced adipose tissue signaling[4]. In women with FHA secondary to exercise, leptin levels are typically low. It functions as a permissive signal to the reproductive axis — its reduction effectively communicates to the hypothalamus that the energy environment is insufficient to support reproduction[6]. Fasting insulin is frequently reduced in LEA states, mirroring the broader shift toward an energy-conserving endocrine profile[6].

In my clinical practice, leptin and fasting insulin are not part of routine assessment for identifying energy deficiency. These markers are generally not used at the level of primary care in this context. Instead, they are primarily reserved for specific endocrinological indications and are more commonly used in the evaluation of other endocrine disorders rather than for diagnosing low energy availability.

Bone Health Blood Markers

Serum calcium is tightly regulated by parathyroid hormone (PTH) and stays within a narrow range even as skeletal calcium is being actively mobilized. A normal serum calcium does not exclude significant bone resorption stress. Clinicians working with athletes presenting with LEA indicators, menstrual dysfunction, or stress fracture history should consider extending the panel to include the following markers:

25-OH Vitamin D: Vitamin D is essential for intestinal calcium absorption, bone mineralization, and osteocalcin regulation. Evidence demonstrates that when vitamin D falls below 30 ng/mL, PTH increases, triggering elevated osteoclastic activity and calcium mobilization from bone[10]. For female athletes specifically, levels above 50 nmol/L (approximately 20 ng/mL) have been proposed as a reasonable target to support bone health, although optimal levels vary across guidelines and should be individualised[11]. Low vitamin D is common across virtually all female athlete populations, with indoor athletes, those training at northern latitudes, and dark-skinned athletes at particular risk[10]. For a full clinical breakdown of vitamin D testing and interpretation in athletes, see Vitamin D for Athletes: An Evidence-Based Guide to Performance, Injury Risk, and Clinical Interpretation.

Parathyroid Hormone (PTH): Elevated PTH in the context of low or borderline vitamin D signals secondary hyperparathyroidism — a compensatory increase in bone resorptive activity attempting to maintain serum calcium. This pattern warrants both vitamin D optimization and enhanced bone health monitoring.

In my clinical practice, I measure vitamin D fairly routinely in primary care, as it is included in many standard laboratory panels. Parathyroid hormone (PTH) is typically a second-line test, most often assessed when serum calcium or vitamin D levels are abnormal. More advanced bone turnover markers, on the other hand, are generally reserved for specialist care. In my experience, they are mainly used in endocrinology settings or in specific research contexts rather than in routine clinical practice.

Nutritional and Haematological Markers

Alongside the hormonal and bone panels, several nutritional and haematological values complete the RED-S blood work screen:

Ferritin: LEA frequently co-occurs with iron deficiency, as energy restriction reduces micronutrient intake globally. In structured athlete screening frameworks, ferritin below 25 µg/L has been used as a secondary indicator of LEA[12]. Iron deficiency independently impairs aerobic capacity and training adaptation, and female athletes in endurance and aesthetic sports face compounded risk through menstrual blood loss. For a detailed breakdown of ferritin interpretation in athletes — including why “normal” is often not optimal — see Ferritin Levels for Athletes: Reasons Why “Normal” Isn’t Optimal. For the broader iron panel, see Iron Panel Interpretation for Athletes: 7 Proven Insights Every Female Athlete Should Know and Serum Iron vs Ferritin: Which Test Actually Matters for Athletes? For advanced iron assessment, Soluble Transferrin Receptor in Athletes: sTfR as The Advanced Iron Marker is worth reading.

Complete Blood Count (CBC): Haemoglobin and haematocrit provide haematological context and can identify dilutional effects or frank anaemia that compound fatigue symptoms in athletes already compromised by LEA. For a detailed discussion of how haemoglobin changes in runners, see Hemoglobin Levels in Runners: When Low Is Normal vs. Performance-Limiting. Haematocrit interpretation is covered in Hematocrit in Athletes: What Your Blood Really Tells You About Performance, and red blood cell dynamics in RBC Count in Endurance Athletes: What Your Blood Test Is Actually Telling You. One mechanism specifically relevant to female distance runners is mechanical haemolysis — discussed in Footstrike Hemolysis in Distance Runners — which can compound apparent iron deficiency and confound ferritin interpretation. MCV shifts are a useful early indicator of developing iron deficiency; see MCV Changes in Athletes.

Fasting Lipid Panel: Changes in lipid metabolism, including elevated LDL cholesterol, have been described in association with low energy availability[1]. In the Sygo et al. athlete screening framework, LDL above 3.0 mmol/L was included as a secondary LEA indicator[12] — and should be treated as an observed pattern in this context rather than a universally reliable diagnostic marker. An elevated LDL in a lean, apparently healthy female athlete should not be dismissed as purely dietary without considering energy availability status.

Fasting Glucose and Fasting Insulin: In the same structured screening framework — applied to elite female sprinters — fasting blood glucose below 4 mmol/L and fasting insulin below 20 pmol/L were used as secondary indicators of LEA [12], reflecting the metabolic shift toward energy conservation that characterises chronic energy deficiency.

In my clinical practice, these tests are part of the basic laboratory panel that I monitor regularly. In many cases — particularly in athletes and occupational health patients — they are checked on an annual basis. On their own, these tests do not establish a diagnosis of LEA or RED-S. However, changes in these values may reflect underlying physiological stress related to energy deficiency. In that sense, they are best understood as baseline investigations — useful for context, but not diagnostic in isolation.

Summary

The Female Athlete Triad and Relative Energy Deficiency in Sport are fundamentally clinical diagnoses, not laboratory diagnoses. While blood work can provide valuable supporting information, no single biomarker — or even a panel of markers — is sufficient to confirm or exclude the condition.

In practice, the most important diagnostic tools remain a careful anamnesis and clinical assessment, particularly in athletes presenting with high training loads, low energy intake, menstrual dysfunction, and unexplained fatigue or performance decline. Many laboratory values may remain within reference ranges despite clear physiological disruption, while others may show subtle, non-specific changes that only gain meaning in the broader clinical context.

Routine tests such as thyroid function, ferritin, lipids, and vitamin D are best understood as baseline markers, whereas more specialized markers often have limited utility in everyday clinical work. Ultimately, early recognition and intervention — focusing on restoring adequate energy availability — are far more important than extensive laboratory investigation, and often represent the key step in preventing long-term health and performance consequences.

RED-S Blood Work and The Female Athlete Triad: A Screening Guide for Coaches and Clinicians

Table of Contents

Introduction

RED-S Blood Work and Low Energy Availability