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Because of the basic understanding that the hormones produced by the thyroid regulate energy metabolism, althletes and overweight people have self-administered thyroid hormones acquired without a relationship with a physician or having diagnosed hypothyroidism.
Athletes appear to prefer to supplement liothyronine since they understand T3 to be the metabolite directly responsible for energy metabolism. It is believed that by increasing T3 levels the user can accelerate fat loss, however there is complexity within this logic regarding T3 supplementation. It is not recommended to self-administer thyroid medications in the absence of hypothyroidism or condition that results in under-active thyroid production. Normal replacement doses are not effective enough to show noticeable changes in body comp. of healthy individuals who already have good function, and large doses that would be required for reducing weight in euthyroid patients could lead to serious or even life-threatening toxicity.
This is particularly dangerous when thyroid hormones are administered along with sympathetic amines that are also often used for weight loss. Healthy patients self-administering thyroid hormones to promote body composition changes will also risk disrupting their endogenous thyroid production with time needed to recover once the medication is discontinued. If the self-administered dose is not adequate, the healthy patient might experience a decrease in circulating hormones than what is maintained naturally without supplementation.
In addition, liothyronine (T3) itself can cause levels to fluctuate inconsistently due to its short half-life, making supplementation less efficient at maintaining consistent levels than the endogenous metabolism of T4 into T3, since T4 is available in continual supply from the thyroid. Athletes can confirm optimal endogenous thyroid function through a blood screening of multiple hormones, then direct their attention to other body composition influencers that might need improvement.