T3

liothyronine sodium (T3)

Cytomel, Tertroxin (UK), Triostat

Pharmacologic class: Synthetic thyroxine hormone

Therapeutic class: Thyroid hormone replacement

Pregnancy risk category A

FDA Box Warning

• Drug has been used (alone or with other agents) to treat obesity. In euthyroid patients, doses within range of daily hormonal requirements are ineffective for weight loss. Larger doses may cause serious or life-threatening toxicity, particularly when given with sympathomimetic amines (such as those used for anorectic effects).

Action

Synthetic form of triiodothyronine (T₃). Regulates cell growth and differentiation; increases metabolism of lipids, proteins, and carbohydrates; and enhances aerobic mitochondrial function. Also reduces tissue lactic acidosis.

Availability

Injection: 10 mcg/ml in 1-ml vials

Tablets: 5 mcg, 25 mcg, 50 mcg

Indications and dosages

➣ Thyroid hormone replacement in mild hypothyroidism

Adults: All dosages individualized. Initially, 25 mcg P.O. daily; may increase in increments of 12.5 to 25 mcg/day q 1 to 2 weeks. Usual maintenance dosage is 25 to 75 mcg P.O. daily.

➣ Myxedema

Adults: All dosages individualized. Initially, 5 mcg P.O. daily; increase in increments of 5 to 10 mcg/day q 1 to 2 weeks, up to 25 mcg/day. If response still isn't adequate, increase by 5 mcg to 25 mcg P.O. daily q 1 to 2 weeks until desired response occurs. Usual maintenance dosage is 50 to 100 mcg/day P.O.

➣ Myxedema coma

Adults: Initially, 25 to 50 mcg I.V.; after 4 hours, reassess patient's need for subsequent doses (up to 65 mcg in 24 hours). In cardiovascular disease, initial dosage is 10 to 20 mcg I.V.

➣ Simple goiter

Adults: All dosages individualized. Initially, 5 mcg P.O. daily. Increase by 5 to 10 mcg/day q 1 to 2 weeks, up to 25 mcg/day; then increase by 12.5 to 25 mcg P.O. daily q week until desired effect occurs. Usual maintenance dosage is 75 mcg P.O. daily.

Children or elderly adults: Initially, 5 mcg P.O once daily. Increase by 5 mcg q 1 to 2 weeks until desired effect occurs.

➣ T₃ suppression test to distinguish hyperthyroidism from thyroid gland autonomy

Adults: 75 to 100 mcg P.O. daily for 7 days in conjunction with radioactive iodine

Dosage adjustment

• Severe, long-standing hypothyroidism

• Cardiovascular disease

• Psychosis or agitation

• Elderly patients

Contraindications

• Hypersensitivity to drug or its components

• Acute myocardial infarction

• Untreated thyrotoxicosis

• Uncorrected adrenal insufficiency and coexisting hypothyroidism

• Artificial rewarming (I.V. form only)

Precautions

Use cautiously in:

• cardiovascular disease, severe renal insufficiency, uncorrected adrenocortical disorders, diabetes mellitus

• elderly patients

• pregnant or breastfeeding patients.

Administration

• Know that all dosages are highly individualized.

• Administer single oral dose in morning with or without food.

• Injectable form is for I.V. use only. Don't give I.M.

• Infuse each 10-mcg dose over 1 minute.

• Give repeat I.V. doses more than 4 hours but less than 12 hours apart.

• Be aware that in T₃ suppression test, radioactive iodine (¹³¹I) is given before and after 7-day liothyronine course.

Adverse reactions

CNS: insomnia, irritability, nervousness, headache

CV: tachycardia, angina pectoris, hypotension, hypertension, increased cardiac output, arrhythmias, cardiovascular collapse

GI: vomiting, diarrhea, cramps

GU: menstrual irregularities

Metabolic: hyperthyroidism, hyperglycemia

Musculoskeletal: accelerated bone maturation (in children), decreased bone density (with long-term use in women)

Skin: alopecia (in children), diaphoresis

Other: weight loss, heat intolerance

Interactions

Drug-drug. Anabolic steroids, antithyroid drugs, asparaginase, barbiturates, carbamazepine, chloral hydrate, clofibrate, corticosteroids, danazol, estrogens, fluorouracil, heparin (with I.V. use), lithium, methadone, mitotane, oxyphenbutazone, perphenazine, phenylbutazone, phenytoin, propranolol, salicylates (large doses), sulfonylureas: altered thyroid function test results

Anticoagulants: increased anticoagulant action

Beta-adrenergic blockers (selected): impaired beta blocker action

Cardiac glycosides: decreased cardiac glycoside blood level

Cholestyramine, colestipol: liothyronine inefficacy

Theophyllines: decreased theophylline clearance

Drug-diagnostic tests. Thyroid function tests: altered values

Drug-food. Foods high in iron or fiber, soybeans: decreased drug absorption

Patient monitoring

See Monitor for evidence of overdose, including signs and symptoms of hyperthyroidism (weight loss, cardiac symptoms, and abdominal cramps).

• In patients with Addison's disease or diabetes mellitus, assess for evidence that these conditions are worsening. In diabetic patients, also monitor blood glucose level.

• Monitor vital signs and ECG routinely.

• Check thyroid and liver function tests.

Patient teaching

• Teach patient to take in morning with or without food.

• Explain that patient may require lifelong therapy and will need to undergo regular blood testing.

• Caution patient to avoid driving and other hazardous activities until he knows how drug affects concentration and alertness.

• Inform parents that hair loss may occur in children during first few months but that this effect is usually transient.

• As appropriate, review all other significant and life-threatening adverse reactions and interactions, especially those related to the drugs, tests, and foods mentioned above.

T₃

Symbol for 3,5,3'-triiodothyronine.

SLC25A5

A gene on chromosome Xq24 that encodes a mitochondrial solute carrier protein, which functions as a gated pore that translocates ADP from the mitochondrial matrix into the cytoplasm. SLC25A5 forms a
homodimer embedded in the inner mitochondria membrane. Suppressed SLC25A5 expression induces apoptosis and inhibits tumour growth.

T₃

Triiodothyronine, see there.