T3 and T4, as mentioned in this study which references the 1999 publication of the British National Formulary states that 10mcg of T3 (as liothyronine) is bioequivalent to 50mcg T4 (as levothyroxine).
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.
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. and a similar lack of results seen with a higher dose substitution and lower doses. These results are in contrast to an earlier study which showed benefit substituting 50mcg T4 for 12.5mcg T3 which appears to stand alone.
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 whereas others suggest it does not exist for those with normal thyroid function. It is a contested field, as some studies do exist that suggest a relation even in those with normal thyroid function. These discrepancies may be due to the highly variable 'normal range' of thyroid hormones.
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. 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. Another study done with 40mcg daily found greater losses in BMI in the groups with T3 supplementation.
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.
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.
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 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.
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.
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. These results have been replicated a short-term metabolic ward study at 100mcg daily, in four divided dosages.
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.
Thyroid hormone status is correlated with cognition, and a lower level of thyroid hormones is related to depressive-like symptoms.
As mentioned in the 'conjunction' section, 15mcg of T3 substitute for T4 does not appear to be any different from T4 in this regard.