A reduced resting metabolic rate (RMR) ratio and suppressed total triiodothyronine (TT₃) have been demonstrated to reflect metabolic compensation to chronic energy deficiency. However, it is unknown whether the relationship between RMR ratio and TT₃ remains constant over time.

To examine the relationship between RMR ratio and TT₃ in free‐living exercising, ovulatory, weight‐stable women (n = 14) for a 12‐month observational period.

Dual‐energy X‐ray absorptiometry (DXA) and indirect calorimetry provided data on anthropometrics and energy expenditure. Harris‐Benedict_, DXA, and Cunningham (1980 and 1991) equations estimated RMR and RMR ratio (measured RMR/predicted RMR). Repeated measures analysis assessed changes over time (ANOVA and Friedman). Generalized linear modeling tested whether RMR ratio threshold predicted TT₃ > 73.2 ng/dL or TT₃ > 80 ng/dL over 12‐months.

Women were 25.9 ± 5.4 years, weighed 59.6 ± 5.2 kg with BMI 22.3 ± 1.4 kg/m² at baseline, which remained constant throughout the study (weight: P = .523; BMI: P = .511). There was no significant effect of time for RMR (P = .886), TT₃ (P = .890), energy availability (P = .212), and RMR ratio (Harris‐Benedict: P = .852; DXA: P = .607; Cunningham₁₉₈₀: P = .754; Cunningham₁₉₉₁: P = .739). When TT₃ > 73.2 ng/dL, each RMR ratio threshold (Harris‐Benedict: P = .021; DXA: P = .019; Cunningham₁₉₈₀: P = .019; Cunningham₁₉₉₁: P = .016) significantly predicted participants as energy replete; however, when using a more lenient clinical TT₃ threshold of >80 ng/dL, only the DXA ratio threshold yielded a significant prediction of TT₃ (P < .001).

The relationship between RMR ratio and TT₃ remains significant and consistent over time in free‐living exercising women, validating the use of RMR ratio for the longitudinal characterization of energetic status in this population (ie, prospective serial monitoring).