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T3, or triiodothyronine, is one of the two circulating Thyroid Hormones and is the more metabolically active one (relative to T4). Used as therapy for hypothyroidics, T3 may hold some promise as being a short-term fat burner and cognitive enhancer vicariously through the effects of thyroid hormones.

Our evidence-based analysis on t3 features 26 unique references to scientific papers.

Research analysis led by and reviewed by the Examine team.
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Research Breakdown on T3

1Comparing and Contrasting T3 and T4

T3 and T4, as mentioned in this study[4] which references the 1999 publication of the British National Formulary[5] states that 10mcg of T3 (as liothyronine) is bioequivalent to 50mcg T4 (as levothyroxine).

1.1Against Each other

The fat loss effects of T3 supplementation appear to be more effective than T4 supplementation at weight loss, although studied in hypothyroidics and dosed in accordance to TSH levels, at an average dose of 0.57mcg/kg bodyweight.[6]

1.2In conjunction with each other

The combination of T3/T4 (in which 50mcg of T4 was exchanged for 10mcg T3) in comparison to straight T4 supplementation had no significant differences in terms of subjective well-being and treatment of hypothyroidism.[4] and a similar lack of results seen with a higher dose substitution[7][8] and lower doses.[9][10][11] These results are in contrast to an earlier study which showed benefit substituting 50mcg T4 for 12.5mcg T3[12] which appears to stand alone.

2Effects on Fat Metabolism

T3 (or more specifically, thyroid hormones in general) are investigated for their interactions with fat mass since, in survey research, thyroid activity may be related to fat mass and BMI.

Some studies suggest a correlation[13] whereas others suggest it does not exist for those with normal thyroid function.[14] It is a contested field, as some studies do exist that suggest a relation even in those with normal thyroid function[15][16][17]. These discrepancies may be due to the highly variable 'normal range' of thyroid hormones.

2.1Interventions on people without (Diagnosed) hypothyroidism

In fasting women, T3 supplementation at 20mcg four times daily acts to preserve a drop in serum T3 levels. In a fed state, this study reported levels of 3.34+/-0.23 nmol/L, which dropped to 0.64+/-0.04nmol/L after a week of fasting but were preserved at 2.9+/-0.3nmol/L.[18] Although both groups experienced a 13% drop in metabolic rate after one week, the T3 group experienced a 5% increase in the second week where control continued to drop.[18] Another study done with 40mcg daily found greater losses in BMI in the groups with T3 supplementation.[19]

In a study on a drastically caloric reduced diet (200kcal) in morbidly obese persons, T3 supplementation at 150mcg was able to preserve the rate of weight loss after two weeks where control had an attenuation in the rate of weight loss.[20]

One study found that the increased rate of weight loss was equivalent to 92g body weight per day, although it did not say what this tissue was.[21]

2.2Animal studies

A recent rat study found that the increased expression of thyroid-hormone receptor beta (T3's receptor) that is increased during caloric restriction is not abolished by T3 supplementation[22] and that T3 supplementation leads to increased leptin gene expression; although circulating leptin levels and overall fat mass were unaffected. Higher doses of T3 (25mcg per 100g rat bodyweight) suppress the expression of these genes.[23]

Overall, the data seem to suggest that T3 does not necessarily spike metabolic rate but that it minimizes a decline seen in metabolic rate due to prolonged caloric restriction. The studies available are done mostly on obese persons though, and there is a lack of data on normal-weight people on hypocaloric diet regimens.

3Effects on Muscle Tissue

In obese subjects, a reduction in urinary nitrogen has been reported with T3 supplementation, and was additive to fasting's reduction in urinary nitrogen; suggesting a muscle preserving effect.[24] These results have been replicated a short-term metabolic ward study at 100mcg daily, in four divided dosages.[25]

However, at least one study has noted an increase in urinary nitrogen after four weeks of usage, in doses ranging from 0.36-1.01mcg/kg bodyweight.[21]

4Effects on Cognition

Thyroid hormone status is correlated with cognition, and a lower level of thyroid hormones is related to depressive-like symptoms.[26]

4.1Human Interventions

As mentioned in the 'conjunction' section, 15mcg of T3 substitute for T4 does not appear to be any different from T4 in this regard.[9]


  1. ^ Bach-Huynh TG, et al. Timing of levothyroxine administration affects serum thyrotropin concentration. J Clin Endocrinol Metab. (2009)
  2. ^ Bolk N, et al. Effects of evening vs morning levothyroxine intake: a randomized double-blind crossover trial. Arch Intern Med. (2010)
  3. ^ Rajput R, Chatterjee S, Rajput M. Can Levothyroxine Be Taken as Evening Dose? Comparative Evaluation of Morning versus Evening Dose of Levothyroxine in Treatment of Hypothyroidism. J Thyroid Res. (2011)
  4. ^ a b Walsh JP, et al. Combined thyroxine/liothyronine treatment does not improve well-being, quality of life, or cognitive function compared to thyroxine alone: a randomized controlled trial in patients with primary hypothyroidism. J Clin Endocrinol Metab. (2003)
  5. ^ British National Formulary Website.
  6. ^ Celi FS, et al. Metabolic effects of liothyronine therapy in hypothyroidism: a randomized, double-blind, crossover trial of liothyronine versus levothyroxine. J Clin Endocrinol Metab. (2011)
  7. ^ Sawka AM, et al. Does a combination regimen of thyroxine (T4) and 3,5,3'-triiodothyronine improve depressive symptoms better than T4 alone in patients with hypothyroidism? Results of a double-blind, randomized, controlled trial. J Clin Endocrinol Metab. (2003)
  8. ^ Rodriguez T, et al. Substitution of liothyronine at a 1:5 ratio for a portion of levothyroxine: effect on fatigue, symptoms of depression, and working memory versus treatment with levothyroxine alone. Endocr Pract. (2005)
  9. ^ a b Clyde PW, et al. Combined levothyroxine plus liothyronine compared with levothyroxine alone in primary hypothyroidism: a randomized controlled trial. JAMA. (2003)
  10. ^ Siegmund W, et al. Replacement therapy with levothyroxine plus triiodothyronine (bioavailable molar ratio 14 : 1) is not superior to thyroxine alone to improve well-being and cognitive performance in hypothyroidism. Clin Endocrinol (Oxf). (2004)
  11. ^ Valizadeh M, et al. Efficacy of combined levothyroxine and liothyronine as compared with levothyroxine monotherapy in primary hypothyroidism: a randomized controlled trial. Endocr Res. (2009)
  12. ^ Bunevicius R, et al. Effects of thyroxine as compared with thyroxine plus triiodothyronine in patients with hypothyroidism. N Engl J Med. (1999)
  13. ^ Knudsen N, et al. Small differences in thyroid function may be important for body mass index and the occurrence of obesity in the population. J Clin Endocrinol Metab. (2005)
  14. ^ Manji N, et al. Lack of association between serum TSH or free T4 and body mass index in euthyroid subjects. Clin Endocrinol (Oxf). (2006)
  15. ^ Shon HS, et al. Free T4 is negatively correlated with body mass index in euthyroid women. Korean J Intern Med. (2008)
  16. ^ Iacobellis G, et al. Relationship of thyroid function with body mass index, leptin, insulin sensitivity and adiponectin in euthyroid obese women. Clin Endocrinol (Oxf). (2005)
  17. ^ De Pergola G, et al. Free triiodothyronine and thyroid stimulating hormone are directly associated with waist circumference, independently of insulin resistance, metabolic parameters and blood pressure in overweight and obese women. Clin Endocrinol (Oxf). (2007)
  18. ^ a b Nair KS, et al. Effect of triiodothyronine on leucine kinetics, metabolic rate, glucose concentration and insulin secretion rate during two weeks of fasting in obese women. Int J Obes. (1989)
  19. ^ Pasquali R, et al. Effect of 'physiological' doses of triiodothyronine replacement on the hormonal and metabolic adaptation to short-term semistarvation and to low-calorie diet in obese patients. Clin Endocrinol (Oxf). (1984)
  20. ^ Koppeschaar HP, Meinders AE, Schwarz F. The effect of a low-calorie diet alone and in combination with triiodothyronine therapy on weight loss and hypophyseal thyroid function in obesity. Int J Obes. (1983)
  21. ^ a b Abraham RR, et al. The effects of triiodothyronine on energy expenditure, nitrogen balance and rates of weight and fat loss in obese patients during prolonged caloric restriction. Int J Obes. (1985)
  22. ^ Luvizotto RA, et al. Administration of physiologic levels of triiodothyronine increases leptin expression in calorie-restricted obese rats, but does not influence weight loss. Metabolism. (2010)
  23. ^ Luvizotto RA, et al. Supraphysiological triiodothyronine doses diminish leptin and adiponectin gene expression, but do not alter resistin expression in calorie restricted obese rats. Horm Metab Res. (2011)
  24. ^ Byerley LO, Heber D. Metabolic effects of triiodothyronine replacement during fasting in obese subjects. J Clin Endocrinol Metab. (1996)
  25. ^ Wilson JH, Lamberts SW. The effect of triiodothyronine on weight loss and nitrogen balance of obese patients on a very-low-calorie liquid-formula diet. Int J Obes. (1981)
  26. ^ Jackson IM. The thyroid axis and depression. Thyroid. (1998)