Formulary
Levothyroxine: Uses, Dosing, Side Effects and Indian Brand Names
Levothyroxine is the standard thyroid hormone replacement for all causes of hypothyroidism. It is a synthetic T4 prohormone converted peripherally to active T3, requiring empty-stomach administration and slow dose titration in elderly and cardiac patients.
MedNext Academy | 5 min read
Clinically reviewed by Awaiting clinical review
Levothyroxine: Uses, Dosing, Side Effects and Indian Brand Names
Levothyroxine is the standard thyroid hormone replacement for all causes of hypothyroidism. It is a synthetic T4 prohormone converted peripherally to active T3, requiring empty-stomach administration and slow dose titration in elderly and cardiac patients.
NEET PG High-Yield: ALWAYS correct adrenal insufficiency BEFORE starting levothyroxine (precipitates adrenal crisis). Congenital hypothyroidism screening and immediate treatment is a perennial question. TSH is the monitoring parameter (wait 6-8 weeks after dose change). In pregnancy, levothyroxine dose needs to be INCREASED by 25-50%. Levothyroxine should not be taken with calcium or iron (absorption interference).
Clinical overview
Levothyroxine is the drug of choice for hypothyroidism of any cause and is one of the most widely prescribed medications globally. It provides physiological replacement of thyroid hormone with predictable pharmacokinetics, a long half-life (6-7 days), and a stable plasma profile. In India, hypothyroidism is highly prevalent (estimated 10-11% of the adult population in some studies, higher in iodine-deficient regions), making levothyroxine one of the most commonly dispensed drugs. The drug must be taken on an empty stomach, ideally 30-60 minutes before breakfast, as food (especially calcium, iron, soy, and fibre) significantly reduces absorption. Patients should be counselled to maintain consistent timing of administration. TSH is the primary monitoring parameter, checked 6-8 weeks after any dose change. In elderly patients and those with ischaemic heart disease, levothyroxine must be started at very low doses (12.5-25 mcg/day) and titrated slowly every 4-6 weeks, as rapid correction of hypothyroidism can precipitate angina, myocardial infarction, or cardiac arrhythmias by increasing myocardial oxygen demand. Myxoedema coma is a medical emergency treated with IV levothyroxine (or IV liothyronine). Congenital hypothyroidism (cretinism) requires early initiation of levothyroxine within the first 2 weeks of life to prevent irreversible intellectual disability -- a cornerstone of neonatal screening programmes.
Pharmacological class
Levothyroxine belongs to the Thyroid hormone replacement (synthetic T4, levo-isomer of thyroxine) class. Levothyroxine (synthetic T4) is a prohormone that is peripherally converted to the active hormone triiodothyronine (T3) by type 1 and type 2 deiodinase enzymes. T3 enters the cell nucleus and binds to thyroid hormone receptors (TR-alpha, TR-beta), which are nuclear transcription factors. This complex binds to thyroid hormone response elements (TREs) on DNA, modulating gene expression to increase basal metabolic rate, protein synthesis, oxygen consumption, and thermogenesis.
Indian brand names and formulations
Available as: Thyronorm (Abbott India), Eltroxin (GlaxoSmithKline), Thyrox (Macleods Pharmaceuticals), Lethyrox (Sun Pharma).
Tablets: 12.5 mcg, 25 mcg, 50 mcg, 75 mcg, 88 mcg, 100 mcg, 112 mcg, 125 mcg, 150 mcg, 175 mcg, 200 mcg. Oral solution (newer, for patients with absorption issues). IV injection (for myxoedema coma -- limited availability in India).
Regulatory status
Levothyroxine is classified under Schedule H in India under the Drugs and Cosmetics Act, 1940. Schedule H prescription drug. NLEM 2022 listed. Available in a wide range of dosage strengths for precise titration. Requires periodic TSH monitoring.
Indications
- Primary hypothyroidism (most common: Hashimoto's thyroiditis)
- Post-thyroidectomy or post-radioiodine hypothyroidism
- Congenital hypothyroidism (cretinism)
- Myxoedema coma (IV formulation)
- TSH suppression in differentiated thyroid carcinoma
- Subclinical hypothyroidism (when TSH >10 mIU/L or symptomatic)
- Hypothyroidism in pregnancy (dose usually needs 25-50% increase)
Dosing
Adults: 1.6 mcg/kg/day as full replacement dose (typically 50-200 mcg/day). Start: 50 mcg/day in healthy adults; 12.5-25 mcg/day in elderly or cardiac patients, increase by 12.5-25 mcg every 4-6 weeks. Take on empty stomach 30-60 minutes before breakfast. Congenital hypothyroidism: 10-15 mcg/kg/day in neonates (urgent -- start within 2 weeks of birth). Pregnancy: increase dose by 25-50% (monitor TSH monthly). TSH target: 0.5-4.5 mIU/L (general); <2.5 mIU/L (first trimester pregnancy); <0.1 mIU/L (thyroid cancer suppression).
Contraindications
- Untreated adrenal insufficiency (correct with hydrocortisone BEFORE starting levothyroxine -- thyroid hormone increases cortisol metabolism, precipitating adrenal crisis)
- Acute myocardial infarction (relative -- start at very low doses if hypothyroid)
- Thyrotoxicosis (obviously -- except in thyroid cancer suppression after thyroidectomy)
- Uncorrected hypopituitarism (correct cortisol first)
Adverse effects
- Overtreatment effects (iatrogenic hyperthyroidism): tachycardia, palpitations, tremor, weight loss, heat intolerance, diarrhoea
- Angina pectoris and myocardial infarction (in susceptible patients if dose is increased too rapidly)
- Atrial fibrillation (with chronic excess dosing, especially in elderly)
- Osteoporosis (with chronic TSH suppression below normal range, especially in postmenopausal women)
- Insomnia, anxiety, irritability (if over-replaced)
- Hair loss (transient, usually in the first few months of treatment)
Drug interactions
- Calcium and iron supplements -- reduce levothyroxine absorption (separate by at least 4 hours)
- Proton pump inhibitors (omeprazole) -- may reduce absorption by increasing gastric pH
- Cholestyramine, sucralfate, aluminium hydroxide -- bind levothyroxine in the gut
- Warfarin -- levothyroxine enhances warfarin sensitivity; monitor INR when thyroid doses change
- Rifampicin, phenytoin, carbamazepine -- increase levothyroxine metabolism via CYP induction
- Oestrogen (OCP, HRT) -- increases thyroxine-binding globulin, may increase levothyroxine requirement
Pregnancy and lactation
Category A. Safe and essential in pregnancy. Hypothyroidism in pregnancy is associated with miscarriage, pre-eclampsia, placental abruption, preterm delivery, and impaired fetal neurodevelopment. Levothyroxine dose typically needs to increase by 25-50% by the end of the first trimester (due to increased TBG, expanded plasma volume, and placental deiodinase activity). TSH should be monitored monthly. Target TSH: <2.5 mIU/L in the first trimester, <3.0 mIU/L in second and third trimesters.
Exam-style clinical scenario
A newborn screened at 48 hours of age has a TSH of 65 mIU/L (normal <20 mIU/L). Confirmatory free T4 is low. The baby is clinically well with no goitre. What is the management and urgency? Answer: This is congenital hypothyroidism. Levothyroxine must be started IMMEDIATELY at 10-15 mcg/kg/day (typically 37.5-50 mcg/day for a term neonate) to prevent irreversible intellectual disability (cretinism). Treatment within the first 2 weeks of life is critical for normal neurodevelopment. The tablet is crushed and given with breast milk or water. TSH and free T4 are rechecked at 2 and 4 weeks, then every 1-3 months in the first year. Neonatal screening for congenital hypothyroidism is part of the Rashtriya Bal Swasthya Karyakram (RBSK) in India.
Cost in India
INR 60-120 per strip of 30 tablets (50 mcg); INR 80-150 per strip of 30 (100 mcg). Thyronorm is the most commonly prescribed brand in India.
Clinical governance
Author: MedNext Editorial Team. Clinical reviewer: Awaiting clinical review. Jurisdiction: India (Drugs and Cosmetics Act, 1940). Sources: CIMS India, Indian Pharmacopoeia. Publication state: Awaiting clinical review. Correction: Report errors at support@mednext.academy.
Frequently Asked Questions
Why must adrenal insufficiency be corrected before starting levothyroxine?
Thyroid hormones increase the metabolic clearance of cortisol. In a patient with untreated adrenal insufficiency (either primary or secondary), starting levothyroxine accelerates cortisol degradation without the adrenal glands being able to compensate, precipitating an acute adrenal crisis (Addisonian crisis) -- hypotension, shock, and potentially death. Always give hydrocortisone first, then start levothyroxine after adrenal replacement is established.
Why does levothyroxine need to be taken on an empty stomach?
Levothyroxine absorption occurs primarily in the duodenum and jejunum and is optimal in an acidic gastric environment. Food (especially calcium-rich dairy, iron, soy protein, and dietary fibre) binds to levothyroxine or alters gastric pH, reducing absorption by 20-40%. Taking the drug 30-60 minutes before breakfast on an empty stomach with water ensures consistent absorption. Alternative timing (bedtime, 2+ hours after dinner) is acceptable if maintained consistently.
How is myxoedema coma managed?
Myxoedema coma is a medical emergency with a mortality rate of 25-60%. Management: (1) IV levothyroxine loading dose 200-400 mcg, then 50-100 mcg/day IV (or IV liothyronine if rapid action needed), (2) IV hydrocortisone 100 mg stat before or with levothyroxine (to prevent adrenal crisis -- undiagnosed adrenal insufficiency is common), (3) Supportive: passive rewarming (active warming causes vasodilation and shock), ventilation if needed, cautious fluid resuscitation, correction of hypoglycaemia and hyponatraemia.
Inside MedNext for this topic
- 411 MedNext-authored chapters
- 80,000+ MCQ bank
- 15 study modes
- Growing visual cheat sheets
Study modes
- Notes
- MCQ
- Audio
- Video
- Visual
- 3D Anatomy
- Trace
- Flashcards
- Mnemonics
- Image Bank
- Clinical
- Microscopy
- Audio QBank
- Cadaver
- Book Match
Continue reading
Subject HubPharmacology Hub
All pharmacology resources in one place.
Drug SchedulesIndian Drug Schedules
Schedule H, H1, X and G classifications.
Test yourself on Levothyroxine
Challenge yourself with targeted pharmacology MCQs on this drug class. Every explanation links back to the chapter that teaches the concept.
Start drug challengeExplore the full formulary