levothyroxine

Contraindicated in:

Hypersensitivity;
Recent MI;
Hyperthyroidism.

Use Cautiously in:

Cardiovascular disease (initiate therapy with lower doses);
Severe renal impairment;
Uncorrected adrenocortical disorders;
Pedi: Monitor neonates and infants for cardiac overload, arrhythmias, and aspiration during 1st 2 wk of therapy;
Geri: Older adults are extremely sensitive to thyroid hormones; initial dose should be ↓.

Adv. Reactions/Side Effects ⬆ ⬇

Usually only seen when excessive doses cause iatrogenic hyperthyroidism.

CV: angina, arrhythmias, tachycardia

Derm: sweating

Endo: heat intolerance, hyperthyroidism, menstrual irregularities

GI: abdominal cramps, diarrhea, vomiting

Metab: weight loss

MS: accelerated bone maturation in children

Neuro: headache, insomnia, irritability

Interactions ⬆ ⬇

Drug-drug:

Cholestyramine, colesevelam, colestipol, and sodium polystyrene sulfonate may bind to and ↓ absorption of orally administered levothyroxine; administer levothyroxine ≥4 hr prior to these medications or monitor TSH levels.
Phosphate binders, including calcium carbonate, ferrous sulfate, lanthanum carbonate, and sevelemer, may bind to and ↓ absorption of orally administered levothyroxine; administer levothyroxine ≥4 hr apart from these medications.
Absorption may be ↓ by orlistat, proton pump inhibitors, sucralfate, antacids, and simethicone.
Phenobarbital and rifampin may ↓ levels and effectiveness; may need to ↑ levothyroxine dosage.
May ↑ the risk of bleeding with warfarin.
May ↑ requirement for insulin or oral hypoglycemic agents in patients with diabetes.
May ↓ effects of digoxin; may need to ↑ digoxin dosage.
Concurrent use with ketamine may lead to significant hypertension and tachycardia.
↑cardiovascular effects with adrenergics (sympathomimetics).
Biotin may interfere with thyroid function test assays; discontinue biotin or biotin-containing supplements for ≥2 days before assessing TSH and/or T4 levels.

Drug-Food:

Foods or supplements containing calcium, iron, magnesium, or zinc may bind levothyroxine and prevent complete absorption.
Absorption may be delayed when used with grapefruit juice.

Availability ⬆ ⬇

(Generic available)

Tablets: 25 mcg; 50 mcg; 75 mcg; 88 mcg; 100 mcg; 112 mcg; 125 mcg; 137 mcg; 150 mcg; 175 mcg; 200 mcg; 300 mcg
Capsules (Tirosint): 13 mcg; 25 mcg; 50 mcg; 75 mcg; 88 mcg; 100 mcg; 112 mcg; 125 mcg; 137 mcg; 150 mcg; 175 mcg; 200 mcg
Oral solution: 13 mcg/mL; 20 mcg/mL; 25 mcg/mL; 50 mcg/mL; 75 mcg/mL; 88 mcg/mL; 100 mcg/mL; 112 mcg/mL; 125 mcg/mL; 137 mcg/mL; 150 mcg/mL; 175 mcg/mL; 200 mcg/mL
Powder for injection: 100 mcg/vial; 200 mcg/vial; 500 mcg/vial
Solution for injection: 20 mcg/mL; 40 mcg/mL; 100 mcg/mL

Route/Dosage ⬆ ⬇

PO (Adults ): Hypothyroidism: 1.6 mcg/kg once daily; may ↑ by 12.5–25 mcg/day every 4–6 wk until patient clinically euthyroid based on signs/symptoms and TSH levels.
PO Geriatric Patients and Patients with Cardiac Disease): 12.5–25 mcg once daily; may ↑ by 12.5–25 mcg/day every 6–8 wk until patient clinically euthyroid based on signs/symptoms and TSH levels.
PO (Children >12 yr): 2–3 mcg/kg/day (≥150 mcg/day).
PO (Children 6–12 yr): 4–5 mcg/kg/day (100–125 mcg/day).
PO (Children 1–5 yr): 5–6 mcg/kg/day (75–100 mcg/day).
PO (Children 6–12 mo): 6–8 mcg/kg/day (50–75 mcg/day).
PO (Infants 3–6 mo): 8–10 mcg/kg/day (25–50 mcg/day).
PO (Infants 0–3 mo or Infants at Risk for Cardiac Failure): 10–15 mcg/kg/day or 25 mcg/day; may ↑ after 4–6 wk to 50 mcg.
IM IV (Adults ): Hypothyroidism: 50–100 mcg/day as a single dose. Myxedema coma/stupor: 300–500 mcg IV; additional 100–300 mcg may be given on 2nd day, followed by daily administration of smaller doses.
IM IV (Children ): Hypothyroidism: 50–80% of the oral dose.

US Brand Names ⬆ ⬇

Ermeza, Euthyrox, Levo-T, Levoxyl, Synthroid, Thyquidity, Tirosint, Tirosint-SOL, Unithroid

Action ⬆ ⬇

Synthetic form of thyroxine (T₄). Replacement of or supplementation to endogenous thyroid hormones.
Principal effect is increasing metabolic rate of body tissues:
- Promote gluconeogenesis,
- Increase utilization and mobilization of glycogen stores,
- Stimulate protein synthesis,
- Promote cell growth and differentiation,
- Aid in the development of the brain and CNS.

Therapeutic effects:

Replacement in hypothyroidism to restore normal hormonal balance.
Suppression of thyroid cancer.

Classifications ⬆ ⬇

Therapeutic Classification: hormones

Pharmacologic Classification: thyroid preparations

Pharmacokinetics ⬆ ⬇

Absorption: Levothyroxine is variably (40–80%) absorbed from the GI tract.

Distribution: Distributed into most body tissues.

Protein Binding: >99%.

Metabolism/Excretion: Metabolized by the liver and other tissues to active T3. Thyroid hormone undergoes enterohepatic recirculation and is excreted in the feces via the bile.

Half-Life: 6–7 days.

Canadian Brand Names ⬆ ⬇

Time/Action Profile ⬆ ⬇

ROUTE	ONSET	PEAK	DURATION
PO	unknown	1–3 wk	1–3 wk
IV	6–8 hr	24 hr	unknown

Patient/Family Teaching ⬆ ⬇

Explain purpose and side effects of levothyroxine to patient.
Instruct patient to take medication as directed at the same time each day. Take missed doses as soon as remembered unless almost time for next dose. If more than 2–3 doses are missed, notify health care provider. Do not discontinue without consulting health care provider.
- Explain to patient that medication does not cure hypothyroidism; it provides a thyroid hormone supplement. Therapy is lifelong.
- Advise patient to notify health care provider if headache, nervousness, diarrhea, excessive sweating, heat intolerance, chest pain, ↑ HR, palpitations, weight loss >2 lb/wk, or any unusual symptoms occur.
- Caution patient to avoid taking other medications concurrently with thyroid preparations unless instructed by health care provider. Advise patient to take 4 hr apart from antacids, iron, and calcium supplements. Advise patient to stop biotin-containing supplements ≥ 2 days before assessing TSH or T4 levels.
- Instruct patient to inform health care providers of thyroid therapy.
- Advise women of reproductive potential to notify health care provider if pregnancy is planned or suspected or if breastfeeding. Pregnancy may ↑ thyroid requirements. Monitor serum TSH levels and adjust levothyroxine dose accordingly during pregnancy. Since postpartum TSH levels are similar to preconception values, levothyroxine dose should return to prepregnancy dose immediately after delivery.
- Emphasize importance of follow-up exams to monitor effectiveness of therapy. Thyroid function tests are performed at least yearly.
Pedi: Discuss with parents the need for routine follow-up studies to ensure correct development. Inform patient that partial hair loss may be experienced by children on thyroid therapy. This is usually temporary.

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Indications ⬇

Contraind./Precautions ⬆ ⬇