Precursor is a large globular protein called thyroglobulin . Tyrosine residues on thyroglobulin are iodinated. Iodinated tyrosine residues are coupled to form iodothyronines. Thyroglobulin broken down to release iodothyronines as thyroid hormones. The Mechanisms of Body Function, 8th ed., 2001.
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Membrane-bound secretory vesicles, containing hormone, bud off from Golgi apparatus and travel to cell membrane. Diffuses to nearby capillary. Stimulus may be physical or chemical, usually neural or hormonal. Fusion of vesicle membrane with plasma membrane. B. Steroid hormones .