thy·rox·ine (th?-r?k?s?n?, -s?n) also thy·rox·in (-r?k?s?n)n. Abbr. T4 An iodine-containing hormone, C15H11I4NO4, produced by the thyroid gland, that increases the rate of cell metabolism and regulates growth. A synthetic form is used in the treatment of thyroid disorders.[thyr(o)- + ox(y)- + in(dole).]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.thyroxine (?a??r?ksi?n; -s?n) or thyroxinn (Biochemistry) the principal hormone produced by the thyroid gland: it increases the metabolic rate of tissues and also controls growth, as in amphibian metamorphosis. It can be synthesized or extracted from the thyroid glands of animals and used to treat hypothyroidism. Chemical name: tetra-iodothyronine; formula: C15H11I4NO4[C19: from thyro- + oxy-2 + -ine2]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014thy?rox?ine (?a??r?k sin, -s?n) also thy?rox?in (-s?n) n. a hormone of the thyroid gland that regulates the metabolic rate of the body: preparations of it used for treating hypothyroidism. [1915?20; thyr- + ox- + in (dol) e (orig. thought to be a chemical component)] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.thyroxineA hormone made in the thyroid that is important for growth and mental development.Dictionary of Unfamiliar Words by Diagram Group Copyright © 2008 by Diagram Visual Information Limited