The prohormone form of thyroid hormone. Converted to active T3 in peripheral tissues by deiodinase enzymes. Standard first-line treatment for hypothyroidism worldwide. More gradual onset than T3 due to conversion step.
T4 is less commonly used in performance settings than T3 because it requires conversion to the active hormone before producing effects, making the onset slower and the dose-response relationship less predictable. Some athletes prefer it for this reason: the conversion step provides a buffer that makes acute overdose effects less likely. It is used ...
T4 is sometimes used in place of T3 in fat loss stacks, particularly in extended prep phases where the slower onset and less acute action is preferred. It appears in the same types of stacks as T3: alongside Clenbuterol for metabolic acceleration and within anabolic protocols that include muscle-sparing compounds to offset catabolism. It is occasio...
The risks are similar to T3 with the addition of variability in conversion efficiency. Some individuals convert T4 to T3 more efficiently than others, and stress, nutritional deficiencies, and certain medications can impair conversion, creating unpredictable results from a fixed dose. The thyroid su...