Can high endurance training with low energy availability lower active T3 while TSH and free T4 stay normal?

In high-endurance athletes, chronic low energy availability (LEA)—where dietary energy intake is insufficient to support both training load and basic physiological functions—triggers a coordinated metabolic downshift to conserve energy. This response is a hallmark of Relative Energy Deficiency in Sport (RED-S).

Clinical evidence

• Selective endocrine patterns: Research demonstrates that LEA consistently leads to a significant decline in active free T3 levels. In contrast, thyroid-stimulating hormone (TSH) and free T4 typically remain within normal or low-normal reference ranges.
• Diagnostic limitations: Standard clinical screenings that rely solely on TSH or free T4 frequently fail to detect this energy-conservation state, as these parameters do not reliably reflect the peripheral metabolic adjustments.

Mechanistic explanations

• Deiodinase regulation: To limit energy expenditure, the body downregulates the peripheral enzymatic conversion of thyroxine (T4) to active triiodothyronine (T3).
• Diversion to reverse T3: Instead of producing active T3, the metabolic pathway diverts T4 conversion toward the inactive isomer, leading to elevated levels of reverse T3 (rT3). This functional adjustment of the hypothalamic-pituitary-thyroid (HPT) axis acts as a survival mechanism rather than primary thyroid pathology.

Clinical implications

• Bone injury risk: Prolonged energy deficits and the accompanying endocrine adaptations directly impair bone remodeling, leading to compromised bone health and a significantly increased risk of injuries like stress fractures.

Bottom line

• High endurance training paired with low energy availability adaptively lowers active T3 to conserve energy while keeping TSH and free T4 normal. Measuring active free T3 and reverse T3 is highly recommended over TSH alone to detect RED-S and mitigate risks to bone health.