Levothyroxine sodium

Generic Medicine Info
Indications and Dosage
Myxoedema coma
Adult: Initially, 200-500 mcg, followed by 100-300 mcg on the 2nd day if necessary, then smaller doses of daily supplements until euthyroid state is achieved or the patient can tolerate oral admin.
Elderly: Lower doses may be needed.

Adult: As replacement therapy: Initially, 50-100 mcg/day may increase by 25-50 mcg at approx 3- to 4-wk intervals until the thyroid deficiency is corrected. Maintenance: 100-200 mcg/day.
Child: Neonates: Initially, 10-15 mcg/kg/day. Adjust dose every 4-6 wk. Neonates w/ thyroxine levels <5 mcg/dl: Initially, 50 mcg/day. Infants and childn: Dose based on body wt and age: 0-3 mth 10-15 mcg/kg/day; 3-6 mth 8-10 mcg/kg/day; 6-12 mth 6-8 mcg/kg/day; 1-5 yr 5-6 mcg/kg/day; 6-12 yr 4-5 mcg/kg/day; >12 yr 2-3 mcg/kg/day. Older childn: To minimise hyperactivity, initially ¼ of the recommended dose and increase by ¼ dose each wk until full replacement dose is reached. Childn who have completed growth and puberty: Initially, 1.7 mcg/kg/day or as per adult dose.
Elderly: >50 yr Initially, 25-50 mcg/day, adjust dose by 12.5-25 mcg increments at 6- to 8-wk intervals.

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 MIU/L for multinodular goitre.

Severe and chronic hypothyroidism
Adult: Initially, 12.5-25 mcg/day, may increase by increments of 25 mcg at 2- to 4-wk intervals.
Child: Initially, 25 mcg/day, may increase by increments of 25 mcg at 2- to 4-wk intervals.
Special Patient Group
Oral Replacement therapy for hypothyroidism Elderly w/ underlying CV disease: <50 yr Initially 25-50 mcg/day, adjust dose by 12.5-25 mcg increments at 6- to 8-wk intervals; ≥50 yr Initially, 12.5-25 mcg/day, adjust dose by 12.5-25 mcg increments at 4- to 6-wk intervals.
Neonates at risk of cardiac failure: Initiate a lower dose of 25 mcg/day, may increase at 4- to 6-wk intervals.
Should be taken on an empty stomach. Take 30 min-1 hr before meals.
Untreated hyperthyroidism; uncorrected adrenal insufficiency; acute MI.
Special Precautions
Patients w/ CV disease (e.g. angina, heart failure, HTN), DM and diabetes insipidus, epilepsy, pre-existing myasthenia syndrome, long-standing hypothyroidism. Elderly, pregnancy and lactation.
Adverse Reactions
Nervousness, excitability, tremor, muscle weakness, fatigue, cramps; sweating, flushing, heat intolerance, headache, fever, insomnia, tachycardia, palpitations, restlessness, anginal pain, HTN, severe depression, difficulty in sleeping, excessive wt loss; menstrual irregularities; diarrhoea, vomiting, psychosis or agitation. Increased bone resorption and reduced bone mineral density, especially in post-menopausal women; elevated LFT.
Potentially Fatal: Thyrotoxic crisis including convulsions, cardiac arrhythmia, heart failure, coma.
IM/IV/Parenteral/PO: A
Monitoring Parameters
Monitor thyroid function test, clinical signs of hypo- and hyperthyroidism, heart rate and BP.
Symptoms: Increased BP, fever, tachycardia, agitation, arrhythmias, anxiety states, hyperkinesis, confusion, neurological complications and coma. Management: Use of activated charcoal and a β-blocker (e.g. propranolol or metoprolol) for tachyarrhythmia. Symptomatic and supportive treatment must also be applied.
Drug Interactions
Reduced absorption w/ iron, antacids, bile acid sequestrants, colestyramine, simeticone, Ca carbonate, sucralfate, cation exchange resins. Reduced tri-iodothyronine serum levels w/ amiodarone and propranolol. Reduced serum levels w/ carbamazepine, phenytoin, phenobarbital, rifampicin, lithium, oestrogens, sertraline. Androgens may decrease levothyroxine-binding globulins serum levels. May alter requirements of antidiabetic drugs. Increased risk of significant HTN and tachycardia w/ ketamine. Increased metabolic demands w/ sympathomimetics (e.g. epinephrine). May increase anticoagulant effect of warfarin.
Food Interaction
Decreased bioavailability and lower serum levels w/ enteral nutrition. Reduced absorption w/ food, soybean infant formula, cottonseed meal, walnuts and dietary fibre.
Description: Levothyroxine Na is a synthetic form of thyroxine which increases the basal metabolic rate (BMR) and the utilisation and mobilisation of glycogen stores and stimulates protein synthesis. It is also involved in normal metabolism, growth and development. These effects are mediated at the cellular level by the thyroxine metabolite, tri-iodothyronine.
Onset: Oral: 3-5 days; IV: 6-8 hr.
Absorption: Variable but adequate from the GI tract. Food decreases absorption. Bioavailability: Oral: 64% (nonfasting state); 79-81% (fasting state). Time to peak plasma concentration: 2-4 hr.
Distribution: Crosses the placenta; minimal amount enters breast milk. Plasma protein binding: >99% (mainly to thyroxine-binding globulin; lesser extent to thyroxine-binding pre-albumin or to albumin).
Metabolism: Hepatic and renal metabolism. Converted to liothyronine and inactive reverse triiodothyronine which undergo further deiodination to inactive metabolites; conjugation also occurs; undergoes enterohepatic recirculation.
Excretion: Via urine (as a free drug, deiodinated metabolites or conjugates); faeces (approx 20%; as free hormone). Elimination half-life: Approx 6-7 days. Prolonged in hypothyroidism and reduced in hyperthyroidism.
Store between 15-30°C. Protect from light and moisture.
MIMS Class
Thyroid Hormones
Anon. Levothyroxine sodium. Lexicomp Online. Hudson, Ohio. Wolters Kluwer Clinical Drug Information, Inc. https://online.lexi.com. Accessed 07/11/2013.

Buckingham R (ed). Levothyroxine sodium. Martindale: The Complete Drug Reference [online]. London. Pharmaceutical Press. https://www.medicinescomplete.com. Accessed 07/11/2013.

Joint Formulary Committee. Levothyroxine sodium. British National Formulary [online]. London. BMJ Group and Pharmaceutical Press. https://www.medicinescomplete.com. Accessed 07/11/2013.

McEvoy GK, Snow EK, Miller J et al (eds). Levothyroxine sodium. AHFS Drug Information (AHFS DI) [online]. American Society of Health-System Pharmacists (ASHP). https://www.medicinescomplete.com. Accessed 07/11/2013.

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