tri·i·o·do·thy·ro·nine (tr???-??d?-th??r?-n?n?, -?-?d??-)n. Abbr. T3 An iodine-containing hormone, C15H12I3NO4, produced by the thyroid gland and similar to but more potent than thyroxine. A synthetic form (liothyronine) is used in the treatment of hypothyroidism.[tri- + iodo- + thyronine, an amino acid (thyr(o)- + -on(e) + -ine).]American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.triiodothyronine (?tra?a????d????a?r??ni?n) n (Biochemistry) an amino acid hormone that contains iodine and is secreted by the thyroid gland with thyroxine, to which it has a similar action. Formula: C15H12I3NO4[C20: from tri- + iodo- + thyro- + -ine2]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014tri?i?o?do?thy?ro?nine (?tra? a??o? do???a? r??nin, -a???d o?-) n. a thyroid hormone, C15H12I3NO4, similar to thyroxine but more potent: preparations of it used in treating hypothyroidism. [1950?55; tri- + iod(ine) + -o- + thyronine (perhaps derivative, with -one, of thyroxine)] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.