Can low energy availability lower T3 and alter stress-axis signaling?

Low energy availability—whether from calorie restriction, fasting, or a mismatch between training load and intake—can produce measurable thyroid and stress-axis adaptations. For a 42-year-old woman, these findings are most clinically relevant when restriction is sustained, symptoms are present, or exercise demands are high.

Thyroid and energy expenditure

• Controlled evidence supports lower circulating T3 during restriction. In the 2-year randomized CALERIE trial, approximately 25% calorie restriction reduced total T3 by 16% at 12 months and 22% at 24 months versus usual intake.
• Acute deficits can also affect thyroid indices: 4 days of very low energy availability reduced free and total T3, while a 5-day 55% energy deficit in healthy women reduced T3 with lower TSH and higher T4 and reverse T3.
• This pattern is compatible with metabolic energy conservation. In CALERIE, energy expenditure fell beyond that expected from weight loss; about 40% of the resting-metabolic-rate reduction was attributed to metabolic adaptation. However, lower T3 cannot yet be considered proven to cause this expenditure reduction.

HPA-axis signaling

• Low energy availability can alter hypothalamic–pituitary–adrenal signaling, including cortisol concentration, diurnal timing, pulsatility, and responses to exercise or ACTH stimulation.
• The direction is not uniform: acute fasting increased serum cortisol in meta-analytic evidence, whereas less severe calorie restriction did not significantly increase it and effects appeared to attenuate over time. One controlled fasting study found an earlier salivary-cortisol peak without altered total cortisol exposure.
• ACTH and DHEA-S responses are inconsistent; randomized evidence found no significant DHEA-S change, with very low certainty.

Clinical interpretation

• Low total or free T3 is a primary IOC REDs risk indicator, but not a stand-alone diagnosis. Cortisol, ACTH, and DHEA-S likewise require contextual interpretation.

Bottom line

• Low energy availability can lower T3 and alter HPA-axis signaling; the T3 change is plausibly part of an energy-conserving adaptation, while individual cortisol-related results are variable and context-dependent.