Published on: 02-Nov-2026
Table of Contents
Persistent fatigue, stalled progress or repeated stress fractures aren’t always overtraining. Thyroid disorders, low energy availability, blood sugar problems, low sex hormones and poor bone health can all mimic it, and most can be identified with simple blood tests or a bone density scan. If symptoms don’t improve with rest, better fueling and sleep, a hormonal evaluation is the next step.
The Thyroid: Your Metabolic Thermostat
The thyroid sets your metabolic pace, affecting heart rate, temperature, muscle function and energy use. Thyroid problems are common and easy to mistake for a hard training block.
- Underactive thyroid: unusual fatigue, muscle weakness or cramps, slow recovery, feeling cold, weight gain and low mood. Training feels harder at the same effort.
- Overactive thyroid: racing heart, unintended weight loss, tremor, anxiety, heat intolerance and poor sleep, often mistaken for pre-competition nerves.
A blood test starting with thyroid-stimulating hormone (TSH) identifies most thyroid problems; unexplained shifts in resting heart rate, energy or weight justify one.
Low Energy Availability and REDs
Relative Energy Deficiency in Sport (REDs) occurs when an athlete doesn’t eat enough to cover both training and the body’s basic needs. According to the 2023 International Olympic Committee consensus statement, the body responds by turning down reproductive hormones, thyroid activity and bone building. Warning signs include:
- Missed or irregular periods in female athletes
- Low libido or unexplained low energy in male athletes
- Frequent illness, slow healing and repeated bone stress injuries
- Mood changes, poor concentration and declining performance despite more training
It isn’t limited to elite or endurance athletes; often it’s simply a gap between training load and fueling. It deserves prompt medical attention because its effects on bone can be long lasting.
Blood Sugar, Insulin and Training
Athletes with type 1 or type 2 diabetes can compete at a high level, but exercise changes blood sugar in ways that need planning. The American Diabetes Association notes that glucose can drop or rise depending on the type, intensity and timing of activity. Continuous glucose monitors help athletes see trends and adjust around workouts. Persistent fatigue after meals, increased thirst or trouble losing weight despite consistent training are worth raising with a doctor, since they can be early signs of insulin resistance.
Testosterone, Estrogen and Recovery
Sex hormones influence muscle repair, bone density, mood and motivation. Heavy training, under-fueling, poor sleep and stress can lower them; persistently low levels slow recovery and raise injury risk. For women, falling estrogen during the menopausal transition can affect sleep, joint comfort, body composition and bone strength. For masters athletes, gradual hormonal shifts are a reason to prioritize strength training, protein and bone health. See why recovery takes longer after 50.
Bone Health: The Injury You Don’t Feel Coming
Vitamin D, calcium, thyroid hormone, sex hormones and cortisol all affect bone density, and low bone density usually has no symptoms until a fracture. Consider a bone health check if you have had more than one stress fracture, a history of missed periods or restricted eating, a family history of osteoporosis, menopause or low testosterone, or long-term steroid use. A DEXA scan, vitamin D level and hormone review can guide treatment early. Recurring stress fractures are also one reason the same running injuries keep coming back.
When to See an Endocrinologist
Your sports medicine physician, athletic trainer or primary care doctor is the first stop. If tests suggest a hormonal cause, or symptoms persist without explanation, a referral to an endocrinology practice is the logical next step. Endocrinologists evaluate thyroid disorders, diabetes, bone health, metabolic concerns, menopause and pituitary conditions, and can work with your sports medicine team. Consider an evaluation for fatigue that rest and nutrition don’t fix, unexplained weight or heart rate changes, absent periods, signs of low testosterone or repeated stress fractures.
Practical Steps for Athletes
- Fuel for the work you do, especially around workouts.
- Protect your sleep; recovery hormones depend on it.
- Track more than your times: energy, mood, sleep, menstrual cycles and resting heart rate.
- Ask about routine blood work, such as thyroid function, vitamin D and blood sugar.
- Don’t self-treat. Supplements and hormone products can disrupt your system further. Get tested first.
Overtraining is real, but so are its mimics, and finding a hormonal cause early protects performance and long-term health.
FAQ
Can hormone problems feel like overtraining?
Yes. Thyroid disorders, low energy availability and low sex hormones can all cause fatigue, slow recovery and declining performance.
What blood tests should athletes ask about?
Common starting points include thyroid function (TSH), vitamin D and blood sugar, with sex hormones and a bone density scan when symptoms point that way.
What is REDs?
Relative Energy Deficiency in Sport, a condition caused by eating too little to support training and basic body functions, which can disrupt hormones and weaken bone.
Sponsored article. Researched and written independently by the Sports Medicine Weekly editorial team. Sports Medicine Weekly LLC does not endorse any product or service described on this site. Content published in Sports Medicine Weekly is for informational purposes only and is not a substitute for professional medical advice. Always seek the advice of your healthcare provider regarding any medical condition.
Resources
- Mountjoy M, et al. 2023 IOC consensus statement on Relative Energy Deficiency in Sport (REDs). Br J Sports Med. 2023. doi:10.1136/bjsports-2023-106994
- Colberg SR, et al. Physical activity/exercise and diabetes: ADA position statement. Diabetes Care. 2016. doi:10.2337/dc16-1728
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