Iodide trapped in the thyroid gland is oxidized to iodine by a process
probably mediated by peroxide enzyme, which is followed by iodinization
of tyrosine to form monoiodotyrosine and diiodotyrosine (DIT). The coupling
of two iodotyrosines to form iodotyronines involves two possible routes.
The two iodotyrosine molecules combine to form one molecule of thyroxine
(T4) or one mono- and one diiodotyrosine combine to form one molecule
of triiodotyronine (T3). T3 is the predominant hormone secreted by the thyroid
gland and is regarded as an active precursor for T4. Major differences
exist between fish and mammals in the handling of iodine and extrathyroidal
metabolism of T4 and T3 (Higgs et al., 1982).