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Levothyroxine information from DrugsUpdate  

See Available Brands of Levothyroxine in India

P - Caution when used during pregnancy
L - Caution when used during lactation
FI - Food *
LI - Lab *

Levothyroxine, also L-thyroxine, synthetic T4, or 3,5,3',5'-tetraiodo-L-thyronine, is a synthetic form of thyroxine (thyroid hormone). The natural hormone is chemically in the L-form, as is the pharmaceutical agent. Dextrothyroxine (D-thyroxine) briefly saw research as an anticholesterol agent but was pulled due to cardiac side-effects. Common brand names include Thyrax, Euthyrox, Levaxin, L-thyroxine and Eltroxin in Europe; Thyrox in South Asia; Eutirox, Levoxyl and Synthroid in North America. There are also numerous generic versions.

Pharmacodynamics

Pharmacokinetics

Levothyroxine sodium increases the basal metabolic rate (BMR) of carbohydrates, fats and proteins. It is also involved in the regulation and differentiation of cell growth. These effects are mediated at the cellular level by the thyroxine metabolite, tri-iodothyronine.

Onset

Oral: 3-5 days; IV: 6-8 hours.

Absorption
Variable but adequate from the GI tract (oral); increased in fasting state.
Distribution: Crosses the placenta; enters breast milk. Protein-binding: Extensive to thyroxine-binding globulin.

Metabolism
Hepatic and renal; converted to liothyronine and inactive reverse triiodothyronine; undergoes enterohepatic recirculation.

Excretion
Via faeces. Half-life: 6-7 days (euthyroid); prolonged in hypothyroidism and reduced in hyperthyroidism.

Levothyroxine Indications / Levothyroxine Uses

Information Not Available

Levothyroxine Adverse Reactions / Levothyroxine Side Effects

Nervousness, excitability, tremor, muscle weakness, cramps; sweating, flushing, heat intolerance, headache, insomnia, tachycardia, palpitations, angina pectoris, excessive wt loss; menstrual irregularities; diarrhoea, vomiting.

Potentially Fatal: Convulsions, cardiac arrhythmia, heart failure, coma.

Precautions

Information Not Available

Special Precautions

Angina, heart failure; DM; diabetes insipidus; elderly; long-standing hypothyroidism; adrenal insufficiency; myxoedema. Do not use for treatment of obesity or for wt loss. Pregnancy, lactation.

Other Drug Interactions

Reduced absorption with iron, colestyramine, colestipol, aluminium- and magnesium-containing antacids, calcium carbonate, simethicone, sucralfate. May alter requirements of antidiabetic drugs. Reduced efficacy of thyroid replacement therapy with imatinib. Reduced tri-iodothyronine serum levels with amiodarone. Reduced serum levels of thyroxine with carbamazepine, phenytoin, phenobarbital, rifampicin, oestrogens.

Potentially Fatal: Increased therapeutic and toxic effects of levothyroxine and TCAs. May change hypoprothrombinaemic response to warfarin and other oral anticoagulants (markedly increased response during replacement). Increased risk of significant hypertension and tachycardia with ketamine.

Other Interactions

Food Interaction
Decreased bioavailability and lower serum levels of thyroxine with enteral nutrition. Reduced absorption with soybean infant formula, cottonseed meal, walnuts and dietary fibre.

Dosage

Oral
Replacement therapy in hypothyroidism
Adult: Initially, 50-100 mcg daily, may increase by 25-50 mcg at 4-weekly intervals until the thyroid deficiency is corrected. Usual maintenance dose: 100-200 mcg daily. Patient should be evaluated every 6-8 wk to monitor response. Initiate with lower doses at 25-50 mg daily in patients >50 years or patients <60 years with underlying CV disease. Lower initial doses may be considered in patients with subclinical hypothyroidism if treatment is considered necessary.
Child: Neonates: Initially, 10-15 mcg/kg/day. Neonates at risk for cardiac failure: Consider lower doses of 25 mcg/day. Neonates with thyroxine levels <5 mcg/dl: Initially, 50 mcg/day. Adjust dose every 4-6 weeks. Infants and children: Dose based on body weight and age: 0-3 mth: 10-15 mcg/kg/day; 3-6 months: 8-10 mcg/kg/day; 6-12 months: 6-8 mcg/kg/day; 1-5 years: 5-6 mcg/kg/day; 6-12 years: 4-5 mcg/kg/day; >12 years: 2-3 mcg/kg/day. Older children: To minimise hyperactivity, initially ¼ of the recommended dose and increase by ¼ dose each weeks until full replacement dose is reached. Children who have completed growth and puberty: Initially, 1.7 mcg/kg/day as a single dose. Titrate dose every 6 weeks. Average initial dose: About 100 mcg; usual dose: ≤200 mcg/day; dose ≥300 mcg/day is rare and reevaluation should be prompted.
Elderly: >50 years without cardiac disease or <50 years with cardiac disease: Initially, 25-50 mcg/day. Adjust dose every 6-8 weeks as needed. >50 years with cardiac disease: Initially, 12.5-25 mcg/day. Adjust dose by 12.5-25 mcg increments every 4-6 weeks. Elderly patients may require <1 mcg/kg/day.

Oral

Severe and chronic hypothyroidism
Adult: Initially, 12.5-50 mcg/day. Adjust dose in steps of 12.5-25 mcg at 4-weekly intervals.
Child: Initially, 25 mcg/day. Adjust dose by 25 mcg every 2-4 weeks.

Oral

TSH suppression
Adult: For thyrotropin-dependent well-differentiated thyroid cancer: Doses >2 mcg/kg/day may be given as a single dose to suppress TSH to <0.1 MIU/L. For benign nodules and nontoxic multinodular goitre: Target TSH is generally higher at 0.1-0.5 MIU/L for nodules and 0.5-1.0 MIU/L for multinodular goitre.

Intravenous

Myxoedema coma
Adult: Initially, 200-500 mcg, followed by 100-300 mcg on the 2nd day if necessary, then 100-200 mcg daily until euthyroid state is achieved or the patient can tolerate oral admin. Patient with cardiac disease: Consider lower dose.
Elderly: Lower doses may be needed.

Food(before/after)

Information Not Available

List of Contraindications

Levothyroxine and Pregnancy

Caution when used during pregnancy

Category A: Controlled studies in women fail to demonstrate a risk to the foetus in the 1st trimester (and there is no evidence of a risk in later trimesters), and the possibility of foetal harm remains remote.

Levothyroxine and Lactation

Caution when used during lactation

Levothyroxine and Children

Information Not Available

Levothyroxine and Geriatic

Information Not Available

Levothyroxine and Other Contraindications

Untreated hyperthyroidism; uncorrected adrenal failure; recent MI.

Storage

Intravenous
Store at 15-30°C (59-86°F).

Oral

Store at 15-30°C (59-86°F).

Lab interference

Intravenous
Store at 15-30°C (59-86°F).

Oral

Store at 15-30°C (59-86°F).

Levothyroxine brands in India:

Proloid Thyropill

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