Thyroid hormones play a central role in the regulation of metabolism, thermogenesis, and energy homeostasis. Among these hormones, triiodothyronine (T3) and thyroxine (T4) influence basal metabolic rate (BMR) by modulating mitochondrial activity and affecting the transcription of genes involved in oxidative metabolism (1,2). While overt thyroid dysfunction—such as hypothyroidism or hyperthyroidism—is well known to affect metabolic rate, less is understood about the impact of thyroid hormone variations within the clinically defined euthyroid range.

Emerging evidence suggests that even subtle shifts in serum thyroid hormone levels within the normal range may have physiological consequences. For instance, higher normal free T3 levels have been associated with increased energy expenditure and lower body weight in population studies (3,4). Conversely, individuals with lower-normal thyroid hormone levels may exhibit features of reduced metabolism, including fatigue, weight gain, and cold intolerance, despite normal thyroid function tests (5). These observations imply that a continuum exists in thyroid hormone activity even among euthyroid individuals, which may influence metabolic regulation.

The concept of "tissue hypothyroidism" or differential peripheral sensitivity to thyroid hormones has also gained attention, as local tissue metabolism may not always mirror circulating hormone levels (6). Moreover, inter-individual variability in deiodinase activity and thyroid hormone receptor responsiveness may further influence metabolic outcomes (7). These nuances warrant closer examination of thyroid hormone variability within the euthyroid range and its association with metabolic parameters such as BMR.

This study aims to explore whether variations in free T3, free T4, and TSH levels within the euthyroid reference range are associated with differences in basal metabolic rate in healthy adults. Understanding this relationship may offer new insights into personalized metabolic management and early identification of individuals at risk for metabolic dysregulation.

Study Design and Participants

This cross-sectional observational study was conducted at a tertiary care center. A total of 120 clinically euthyroid adults aged 20 to 50 years were recruited through voluntary participation. Euthyroid status was defined as serum thyroid-stimulating hormone (TSH) levels ranging between 0.45 and 4.5 mIU/L, with free triiodothyronine (fT3) and free thyroxine (fT4) levels within laboratory reference ranges. Individuals with a history of thyroid disease, on thyroid-modulating medications, or with chronic systemic illness were excluded.

Clinical and Anthropometric Assessment

Participants underwent a detailed clinical evaluation including measurement of height, weight, and body mass index (BMI). Physical activity level was assessed using the International Physical Activity Questionnaire (IPAQ) and categorized into low, moderate, and high activity levels.

Biochemical Evaluation

After overnight fasting, venous blood samples were collected between 8:00 and 10:00 AM. Serum levels of free T3, free T4, and TSH were measured using a chemiluminescent immunoassay analyzer (ADVIA Centaur XP, Siemens Healthineers). All assays were conducted in duplicate, and internal quality control measures were maintained.

Metabolic Rate Assessment

Basal metabolic rate (BMR) was measured using indirect calorimetry (COSMED Quark RMR, Rome, Italy) under standard resting conditions in a quiet, temperature-controlled room. Participants were instructed to abstain from caffeine, alcohol, and vigorous exercise for 24 hours prior to testing. The measurement lasted 30 minutes, with the first 5 minutes excluded to account for acclimatization.

Statistical Analysis

Participants were stratified into tertiles based on free T3 and free T4 concentrations. Descriptive statistics were presented as mean ± standard deviation (SD). Pearson correlation was used to assess relationships between thyroid hormone levels and BMR. Multiple linear regression analysis was performed to determine independent predictors of BMR, adjusting for age, gender, BMI, and physical activity. A p-value <0.05 was considered statistically significant. Data analysis was conducted using IBM SPSS Statistics version 26.0.

A total of 120 euthyroid participants (64 males and 56 females) with a mean age of 34.7 ± 8.3 years were included in the final analysis. The overall mean body mass index (BMI) was 24.8 ± 3.9 kg/m². The mean serum levels of free T3, free T4, and TSH were 3.4 ± 0.6 pg/mL, 1.2 ± 0.2 ng/dL, and 2.1 ± 0.9 mIU/L, respectively.

Association of Thyroid Hormone Levels with Basal Metabolic Rate

Participants were categorized into tertiles based on free T3 levels. The mean BMR values showed a progressive increase across the tertiles. Individuals in the highest tertile of free T3 (>3.7 pg/mL) exhibited significantly higher BMR values (mean: 1538 ± 115 kcal/day) compared to the lowest tertile (<3.1 pg/mL), which had a mean BMR of 1345 ± 108 kcal/day (p<0.01). A similar, though less pronounced, trend was observed with free T4. TSH levels did not show any statistically significant correlation with BMR (Table 1).

Table 1: Association of Thyroid Hormone Tertiles with Basal Metabolic Rate

Data expressed as mean ± standard deviation. Statistical significance based on ANOVA.

Multivariate linear regression analysis identified free T3 as an independent predictor of BMR (β = 0.31, p = 0.005), even after adjusting for age, sex, BMI, and physical activity. Free T4 was marginally associated with BMR (β = 0.19, p = 0.04), while TSH remained non-significant (β = –0.07, p = 0.41) (Table 2).

Table 2: Multivariate Regression Analysis for Predictors of Basal Metabolic Rate

This study demonstrated a significant association between free T3 levels and basal metabolic rate (BMR) in euthyroid individuals. Even within the clinically normal range, individuals with higher free T3 concentrations exhibited a notably higher BMR, independent of body mass index (BMI), age, sex, and physical activity. These findings suggest that minor variations in thyroid hormone levels within the euthyroid spectrum may have physiological consequences on energy metabolism.

Thyroid hormones, particularly T3, are essential regulators of metabolic activity, influencing oxygen consumption and mitochondrial biogenesis across various tissues (1). The present findings are consistent with earlier studies indicating that serum T3 correlates more closely with metabolic activity than T4 or TSH, due to its greater biological potency and direct cellular action (2,3). T3 exerts its thermogenic effects through nuclear and non-nuclear mechanisms, including enhanced expression of uncoupling proteins and stimulation of Na+/K+ ATPase (4,5).

Our results also align with large epidemiological surveys suggesting that individuals in the upper-normal range of free T3 have lower body weight and increased resting energy expenditure (6,7). A study by Roef et al. found that free T3 levels, even within the reference range, were positively associated with markers of metabolic syndrome, indicating a possible adaptive increase in metabolic rate to counteract metabolic imbalance (8). Similarly, Knudsen et al. reported that small shifts in thyroid hormones within the normal range could predict future weight changes (9).

Interestingly, TSH did not show a significant correlation with BMR in our cohort, reinforcing the view that TSH, while critical for diagnosing thyroid disorders, may be less useful for assessing subtle metabolic variations within euthyroid individuals (10). This dissociation between TSH and metabolic indices has been observed in previous studies, suggesting that T3-based assessments may offer better insights into metabolic status (11).

Our multivariate analysis further confirmed the independent predictive value of free T3 for BMR. This underscores the importance of individual set-points and tissue-level thyroid hormone sensitivity, which may vary despite similar circulating hormone levels (12). Variations in deiodinase activity, which convert T4 to active T3, may also contribute to this individual variability (13). The concept of “low tissue T3 syndrome,” often observed in non-thyroidal illness, supports the idea that circulating thyroid hormone levels may not fully reflect tissue metabolic activity (14).

In addition, free T4 was marginally associated with BMR, which is in line with some studies but not others (15). This may be due to the fact that T4 serves mainly as a prohormone and requires intracellular conversion to T3 to exert metabolic effects. The weaker association between free T4 and BMR in our study likely reflects this physiological limitation.

Limitations of our study include its cross-sectional design, which precludes causality assessment. Moreover, we did not measure reverse T3 or markers of tissue-specific deiodinase activity, which may have provided deeper mechanistic insights. Despite these limitations, the use of indirect calorimetry and robust statistical adjustments enhances the credibility of our findings.

In conclusion, our study highlights those variations in free T3, even within the euthyroid range, are associated with significant alterations in metabolic rate. These results suggest a potential role for thyroid hormone profiling in metabolic assessments, even in the absence of overt thyroid dysfunction.

1. Yang S, Lai S, Wang Z, Liu A, Wang W, Guan H. Thyroid Feedback Quantile-based Index correlates strongly to renal function in euthyroid individuals. Ann Med. 2021;53(1):1945–55. doi:10.1080/07853890.2021.1993324. PMID: 34726096.
2. Xiao Q, Zhang Z, Ji S, Li M, Zhang B, Xu Q, et al. Association between oxidative balance score and thyroid function and all-cause mortality in euthyroid adults. Sci Rep. 2025;15(1):6817. doi:10.1038/s41598-025-90491-5. PMID: 40000721.
3. Turgay B, Şükür YE, Ulubaşoğlu H, Sönmezer M, Berker B, Atabekoğlu C, et al. The association of thyroid stimulating hormone levels and intrauterine insemination outcomes of euthyroid unexplained subfertile couples. Eur J Obstet Gynecol Reprod Biol. 2019;240:99–102. doi:10.1016/j.ejogrb.2019.06.022. PMID: 31238206.
4. Zhang Y, Chang Y, Ryu S, Cho J, Lee WY, Rhee EJ, et al. Thyroid hormone levels and incident chronic kidney disease in euthyroid individuals: the Kangbuk Samsung Health Study. Int J Epidemiol. 2014;43(5):1624–32. doi:10.1093/ije/dyu126. PMID: 25011453.
5. Barasch E, Gottdiener J, Buzkova P, Cappola A, Shah S, DeFilippi C, et al. Association of thyroid dysfunction in individuals 65 years of age with subclinical cardiac abnormalities. J Clin Endocrinol Metab. 2024;109(10):e1847–e1856. doi:10.1210/clinem/dgae001. PMID: 38183678.
6. Huang B, Yang S, Ye S. Association between thyroid function and nonalcoholic fatty liver disease in euthyroid type 2 diabetes patients. J Diabetes Res. 2020;2020:6538208. doi:10.1155/2020/6538208. PMID: 32964054.
7. Nagel A, Spinneker A, Neuhäuser-Berthold M. Association of thyroid-stimulating hormone with resting energy expenditure in euthyroid elderly subjects: a cross-sectional study. Ann Nutr Metab. 2016;68(1):12–8. doi:10.1159/000441625. PMID: 26555616.
8. Vissenberg R, Fliers E, van der Post JA, van Wely M, Bisschop PH, Goddijn M. Live-birth rate in euthyroid women with recurrent miscarriage and thyroid peroxidase antibodies. Gynecol Endocrinol. 2016;32(2):132–5. doi:10.3109/09513590.2015.1092513. PMID: 26430806.
9. Papini E, Attanasio R, Žarković M, Nagy EV, Negro R, Perros P, et al. Thyroid hormones for euthyroid patients with simple goiter growing over time: a survey of European thyroid specialists. Endocrine. 2025;87(1):262–72. doi:10.1007/s12020-024-04002-z. PMID: 39217207.
10. Roef GL, Taes YE, Kaufman JM, Van Daele CM, De Buyzere ML, Gillebert TC, et al. Thyroid hormone levels within reference range are associated with heart rate, cardiac structure, and function in middle-aged men and women. Thyroid. 2013;23(8):947–54. doi:10.1089/thy.2012.0471. PMID: 23339744.
11. Chen J, Zhou W, Pan F, Cui W, Li M, Hu Y. Age-related change in thyroid-stimulating hormone: a cross-sectional study in healthy euthyroid population. Endocr J. 2018;65(11):1075–82. doi:10.1507/endocrj.EJ18-0113. PMID: 30068892.
12. Negro R, Mestman JH. Thyroid disease in pregnancy. Best Pract Res Clin Endocrinol Metab. 2011;25(6):927–43. doi:10.1016/j.beem.2011.07.010. PMID: 22115167.
13. García-Alfaro P, Pérez-López FR, Sulé MA, Rodríguez I. Evaluation of the association between thyroid-stimulating hormone with handgrip strength and dynapenia in euthyroid postmenopausal women. Menopause. 2025;32(4):331–6. doi:10.1097/GME.0000000000002499. PMID: 39874448.
14. Gu L, Yang J, Gong Y, Ma Y, Yan S, Huang Y, et al. Lower free thyroid hormone levels are associated with high blood glucose and insulin resistance; these normalize with metabolic improvement of type 2 diabetes. J Diabetes. 2021;13(4):318–29. doi:10.1111/1753-0407.13118. PMID: 32981234.
15. Birck MG, Janovsky CCPS, Goulart AC, Meneghini V, Pititto BA, Sgarbi JA, et al. Associations of TSH, free T3, free T4, and conversion ratio with incident hypertension: results from the prospective Brazilian Longitudinal Study of Adult Health (ELSA-Brasil). Arch Endocrinol Metab. 2024;68:e230301. doi:10.20945/2359-4292-2023-0301. PMID: 38739525.