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triiodothyronine (T3, liothyronine, Cytomel)

Biochemistry: - T3 is formed (via activity of thyroid peroxidase) & stored as an iodinated residue of thyroglobulin - normal thyroid gland makes 8 ug of T3/day - T3 is preferentially synthesized by the thyroid when availability of iodide is low - monodeiodination of T4, mainly by the liver, normally produces 32 ug of T3/day. - 99.7% of T3 in protein bound - T3 is more active than T4, estimates range form 3-100 fold more active - treatments that decrease peripheral conversion of T4 -> T3: a) propranolol b) propylthiouracil (methimazole does not) c) glucocorticoids (dexamethasone 4-8 mg/day) Pathology: - diminished peripheral conversion of T4 -> T3 can result in euthyroid sick syndrome Pharmacology: Tradename: Cytomel. Indications: - hypothyroidism, thyroid replacement - cretinism, myxedema, myxedema coma [4] - management of non-toxic goiter - chronic lymphocytic thyroiditis - adjunct agent in thyroiditis - thyroid carcinoma [4] - diagnostic aid Start 25 ug PO QD, max 100 ug/day. Tabs: 5, 25, 50 ug. Adverse effects: - uncommon (< 1%) - palpitations, nervousness, sweating, tachycardia, headache, cardiac arrhythmias, tremors, insomnia, fever, hair loss, weight loss, increased appetite, diarrhea, abdominal cramps, changes in menstrual cycle, constipation, muscle aches, chest pain, shortness of breath, clumsiness - Toxicity: - serum TSH low - serum free T4 is not elevated Drug interactions: warfarin Mechanism of action: 1) synthetic hormone identical to T3 2) thyroid hormones increase metabolic rate of tissues 3) involved in regulation of growth & differentiation

Interactions

drug interactions

Related

reverse T3; reverse triiodothyronine T3 total in serum/plasma thyroglobulin; Tg (TG) thyroid peroxidase triiodothyronine [T3] (free) in serum triiodothyronine [T3] receptor or erbA proto-oncogene protein

General

thyroid agent thyroid hormone thyronine

Properties

SIZE: MW = 651 G/M MISC-INFO: elimination route LIVER pregnancy-category A safety in lactation ?

Database Correlations

PUBCHEM correlations

References

  1. Drug Information & Medication Formulary, Veterans Affairs, Central California Health Care System, 1st ed., Ravnan et al eds, 1998 - not on National VA formulary
  2. Kaiser Permanente Northern California Regional Drug Formulary, 1998
  3. Medical Knowledge Self Assessment Program (MKSAP) 11, American College of Physicians, Philadelphia 1998
  4. Deprecated Reference

Component-of

levothyroxine/triiodothyronine thyroxine/triiodothyronine (Euthroid, Thyrolar)