Amino acid which takes part in the formation of thyroxine is
Tyrosine
The thyroid gland is composed of follicles and stromal tissues. Each thyroid follicle is composed of follicular cells, enclosing a cavity. These follicular cells synthesise two hormones, tetraiodothyronine or thyroxine (T4) and triiodothyronine (T3). The follicular cells add the amino acid tyrosine to a glycoprotein called thyroglobulin. This thyroglobulin is then secreted into the follicular lumen. The thyroglobulin here is actually the precursor for the thyroid hormones. At the junction of the follicular cells and the follicular lumen the tyrosine residues on thyroglobulin get iodised, meaning iodine ions are attached to it. Depending on the number of iodines attached to the tyrosine residues, they either become monoiodotyrosine or diiodotyrosine molecules. These molecules then combine with each other in different combinations to either form T3 which is triiodothyronine or T4 which is thyroxine.