Oskela
Get started

Drug Reference

levothyroxine

Thyroid hormone· Brands: Synthroid, Levoxyl, Eltroxin, Euthyrox, TirosintIVPO

Mechanism of action
Levothyroxine is a synthetic form of thyroxine (T4), the main hormone secreted by the thyroid gland. It is converted in peripheral tissues to triiodothyronine (T3), which binds to thyroid hormone receptors in the nucleus of cells. This binding regulates gene expression, influencing metabolism, growth, and development. The effects include increased basal metabolic rate, protein synthesis, and carbohydrate and fat metabolism.
Pharmacokinetics
Levothyroxine is well absorbed orally, but absorption is variable and reduced by food, particularly soy, and by medications such as calcium and iron. It is highly protein-bound (>99%) to thyroxine-binding globulin. The half-life is approximately 7 days, allowing once-daily dosing. It undergoes deiodination in the liver, kidney, and other tissues; the metabolites are excreted in bile and urine.
Indications & dosing

Primary hypothyroidism

Start 25-50 mcg daily, increase by 25-50 mcg every 4-6 weeks; usually 1.6 mcg/kg/day.

  • adult · PO: 25–200 microgram daily · Start 25-50 mcg daily, increase by 25-50 mcg every 4-6 weeks; usually 1.6 mcg/kg/day.

Myxedema coma

Give 200-400 mcg IV loading dose, then switch to oral maintenance.

  • adult · IV: 200–400 microgram once · Give 200-400 mcg IV loading dose, then switch to oral maintenance.

Congenital hypothyroidism

Initiate promptly; adjust to maintain normal TSH and free T4.

  • neonatal · PO: 10–15 microgram/kg/day daily · Initiate promptly; adjust to maintain normal TSH and free T4.

Thyroid cancer (TSH suppression)

Dose titrated to maintain TSH in the desired range.

  • adult · PO: 150–300 microgram daily · Dose titrated to maintain TSH in the desired range.

Pediatric hypothyroidism

Dose adjusted per age and weight; monitor growth and development.

  • pediatric · PO: 4–6 microgram/kg/day daily · Dose adjusted per age and weight; monitor growth and development.
Contraindications & precautions
  • Untreated thyrotoxicosis · Exogenous thyroid hormone exacerbates hyperthyroidism.
  • Acute myocardial infarction · Increased metabolic demand may worsen cardiac ischaemia; specialist advice required.
  • Acute myocarditis · May precipitate arrhythmias or heart failure.
  • Uncorrected adrenal insufficiency · Thyroid hormone can precipitate adrenal crisis; give glucocorticoids first.
  • Start with a low dose in elderly patients or those with cardiac disease, and titrate slowly to avoid adverse effects.
  • Monitor serum TSH regularly and adjust dose to maintain TSH within the target range.
  • Administer on an empty stomach, at least 30 to 60 minutes before breakfast, and separate from calcium, iron, and antacids by at least 4 hours.
  • In patients with diabetes, monitor blood glucose as thyroid hormone may increase insulin requirements.
  • During pregnancy, doses may need to increase; monitor thyroid function closely.
  • Avoid over-replacement, as chronic overtreatment increases the risk of atrial fibrillation and osteoporosis.
Adverse effects
  • Palpitations and tachycardia
  • Tremor
  • Anxiety and insomnia
  • Heat intolerance and sweating
  • Diarrhoea
  • Cardiac arrhythmias (e.g. atrial fibrillation)serious
  • Angina pectorisserious
  • Myocardial infarctionserious
  • Osteoporosis (with long-term overtreatment)serious
Interactions
  • omeprazolemoderateMonitor serum TSH and free T4 levels after initiating or discontinuing omeprazole, especially in patients on long-term levothyroxine. Advise patients to take levothyroxine at least 4 hours apart from omeprazole if possible, although gastric pH effects may persist. Adjust levothyroxine dose based on thyroid function tests. Consider alternative acid-suppressants if clinically appropriate, or increase monitoring frequency.
  • ferrous sulfatemoderateSeparate administration of levothyroxine and ferrous sulfate by at least 4 hours. Monitor serum TSH and free T4 levels, especially when starting or stopping iron therapy, and adjust the levothyroxine dose as needed. If separation is not feasible, consider alternative iron preparations or routes, and reassess thyroid function regularly.
  • ferrous fumaratemoderateAdminister levothyroxine and ferrous fumarate at least 4 hours apart to minimise the interaction. Monitor serum TSH and thyroid function tests, particularly when starting or stopping iron therapy, and adjust the levothyroxine dose as necessary. If separation is not feasible, consider alternative iron formulations or consult a specialist.
  • calcium carbonatemoderateAdminister levothyroxine and calcium carbonate at least 4 hours apart to avoid the interaction. This can be achieved by taking levothyroxine on an empty stomach and calcium carbonate with meals or at a different time of day. Monitor TSH levels approximately 6 to 8 weeks after any change in the dosing schedule or the introduction or discontinuation of calcium carbonate, and adjust the levothyroxine dose as needed.
  • amiodaronemoderateMonitor thyroid function tests (TSH, free T4, free T3) at baseline before starting amiodarone, then every 1-3 months during the first year of therapy, and periodically thereafter. Adjust levothyroxine dose based on TSH and free T4 levels. Consider alternative antiarrhythmic agents if thyroid dysfunction is difficult to manage.
  • warfarinmajorMonitor INR closely (e.g., weekly) when levothyroxine is initiated, discontinued, or the dose is changed. Adjust the warfarin dose as needed to maintain the target INR. Advise patients to report signs of bleeding.
Monitoring, pregnancy & toxicity
  • Serum TSH · every 6-8 weeks after dose change, then every 6-12 months when stable
  • Free T4 · with TSH, especially during dose titration
  • Clinical signs of hypothyroidism or hyperthyroidism · at each visit
  • Pregnancy: Levothyroxine is safe and essential during pregnancy. Requirements may increase; monitor thyroid function regularly and adjust dose to maintain euthyroidism.
  • Toxicity: Tachycardia, palpitations, and arrhythmias · For symptomatic overdose, administer beta-blockers and withhold levothyroxine.
  • Toxicity: Tremor, anxiety, insomnia, and heat intolerance · Supportive care and dose reduction or temporary discontinuation.
  • Toxicity: Weight loss, increased appetite, and diarrhoea · Monitor thyroid function and adjust dose; consider specialist advice for severe cases.
Clinical notes
Levothyroxine is used for replacement therapy in hypothyroidism, including congenital hypothyroidism and myxedema coma. Doses are individualized based on TSH levels, age, weight, and cardiac status. Monitoring of TSH and free T4 is essential; the dose is titrated to achieve a euthyroid state. Interactions with antacids, calcium, iron, and certain foods can impair absorption; take on an empty stomach. During pregnancy, requirements often increase; specialist management is advised. Oskela content is for education only, not a substitute for clinical judgment.

Log in to read the full levothyroxine monograph

A free account unlocks the complete drug reference, the interaction checker, prescription practice and every other learning tool.

Log inCreate a free account

BNF; local formulary