Liothyronine
Liothyronine (T3, triiodothyronine sodium)
Cytomel (oral), Triostat (IV)
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Summary
Classes
Common uses
Hypothyroidism (T3 supplement, selected refractory cases)0, Myxedema coma (IV, in addition to T4)0, Bridge between T4 withdrawal and radioiodine scan/treatment in thyroid cancer0
Pharmacy
Starting dose
Hypothyroidism: 5-25 mcg PO daily (start low, titrate slowly); myxedema coma: 5-20 mcg IV q4-12h with T4 loading
Preparations
5, 25, 50 mcg tablets (Cytomel); 10 mcg/mL IV (Triostat)
US FDA Max
100 mcg/d typical
Pharmacology
Routes
Oral, IV
Onset
Hours (faster than T4); peak biologic activity 24-48 hours
Duration
24-72 hours
Half-life
~1 day (much shorter than T4's ~7 days)[1]
Bioavailability
~95% (oral)[1]
Pregnancy
T4 (levothyroxine) is the first-line in pregnancy; T3 is rarely needed.[citation needed]
Legal status
Rx-only in US
Purported mechanism
Liothyronine is bioidentical T3 (triiodothyronine), the active thyroid hormone that binds nuclear thyroid hormone receptors with ~10× the affinity of T4; T4 (levothyroxine) is essentially a prodrug peripherally deiodinated to T3.0 Direct T3 supplementation bypasses peripheral deiodination, useful in myxedema coma (rapid clinical effect needed) and in some refractory hypothyroidism (combination T4+T3, controversial). Short half-life produces fluctuating serum levels with intermittent supraphysiologic peaks — one reason T4 monotherapy remains the practical standard[1].
References
- ↑ 1.0 1.1 1.2 FDA Prescribing Information, Cytomel (liothyronine sodium), Pfizer/King, current revision. https://www.accessdata.fda.gov/drugsatfda_docs/label/2017/010379s055lbl.pdf