Does high endurance training or relative energy deficiency cause a peripheral drop in T3 while TSH and fT4 remain normal?

In endurance athletes and individuals experiencing Relative Energy Deficiency in Sport (RED-S), the body initiates a survival-based metabolic adaptation to prioritize essential physiological functions over performance. This state of low energy availability (LEA)—often defined as intake below 30 kcal/kg of fat-free mass per day—triggers a systematic downshift in the thyroid profile to conserve energy.

Clinical and effectiveness evidence

Research consistently identifies a specific thyroid pattern in endurance athletes characterized by significant reductions in triiodothyronine (T3) levels, even when thyroid-stimulating hormone (TSH) and free thyroxine (fT4) remain within standard clinical reference ranges. This phenomenon, often referred to as "Low T3 Syndrome" or "Non-Thyroidal Illness Syndrome" in an athletic context, represents a functional adaptation to chronic energy deficits rather than a primary thyroid disorder. Studies indicate that while TSH and fT4 are relatively resilient to short-term energy flux, circulating T3 is highly sensitive to energy availability and serves as a marker for the body's metabolic status.

Mechanistic explanations

The downshift in T3 is primarily a peripheral rather than a central process, mediated by the modulation of deiodinase enzymes:

• Deiodinase Inhibition: Low energy availability inhibits Type 1 and Type 2 deiodinases (D1 and D2), which are responsible for converting the pro-hormone T4 into the biologically active T3 in the liver, kidney, and skeletal muscle.
• Metabolic Shunting: Simultaneously, there is an upregulation of Type 3 deiodinase (D3), which diverts thyroid hormone metabolism away from active T3 and toward the production of reverse T3 (rT3), an inactive metabolite.
• Energy Conservation: By reducing bioactive T3, the body effectively lowers its resting metabolic rate (RMR), minimizes thermogenesis, and shifts toward a catabolic state. This serves to bridge the gap between energy intake and the high demands of endurance training.

Bottom line

High endurance training loads combined with inadequate caloric intake reliably produce an adaptive reduction in peripheral T3 to conserve energy. Because TSH and fT4 often remain normal, clinicians must specifically monitor free T3 levels to identify energy deficiency in highly active individuals.