Levothyroxine

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Levothyroxine on Prescriptsy

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Levothyroxine sodium is a synthetic thyroid hormone used to treat hypothyroidism, a condition in which the thyroid gland does not produce enough thyroxine (T4).

It is the standard, first-line treatment for all forms of hypothyroidism and is one of the most frequently prescribed medicines in the United Kingdom.

Levothyroxine is available as tablets in strengths of 25, 50, 75, and 100 micrograms, with oral solution formulations available for patients who have difficulty swallowing tablets.

It is a prescription-only medicine.

This page provides a comprehensive clinical overview of levothyroxine for UK patients and prescribers, including how it works, dosage guidance, monitoring requirements, side effects, important drug interactions, and practical advice for daily use.

Hypothyroidism is a common endocrine disorder that affects approximately 2% of the UK population, with a higher prevalence in women and older adults.

The most common cause in the UK is Hashimoto thyroiditis (autoimmune thyroiditis), in which the immune system attacks and gradually destroys the thyroid gland.

Other causes include thyroidectomy (surgical removal of the thyroid), radioactive iodine treatment for hyperthyroidism or thyroid cancer, certain medications (lithium, amiodarone, immune checkpoint inhibitors), iodine deficiency (uncommon in the UK), and pituitary or hypothalamic disease (secondary or tertiary hypothyroidism).

Without treatment, hypothyroidism causes progressive fatigue, weight gain, cold intolerance, constipation, dry skin, hair loss, depression, cognitive impairment, and, in severe cases, myxoedema coma, a life-threatening emergency.

Important safety information about levothyroxine

Before reading further, note these essential safety points about levothyroxine.

• Levothyroxine is a prescription-only medicine that requires blood test monitoring (TSH and free T4) to ensure correct dosing.
• Take levothyroxine on an empty stomach, at least 30 to 60 minutes before food, with water only. Do not take it with coffee, calcium, or iron.
• Do not switch brands without medical guidance, as bioavailability may differ between formulations.
• Patients with heart disease must start at a low dose (25 micrograms) to avoid cardiac complications.
• Adrenal insufficiency must be treated before starting levothyroxine to avoid adrenal crisis.
• Levothyroxine is safe and essential during pregnancy. The dose usually needs increasing, and TSH should be monitored frequently.

Understanding hypothyroidism

The thyroid gland is a butterfly-shaped organ at the front of the neck that produces two hormones: thyroxine (T4) and triiodothyronine (T3).

These hormones regulate the metabolic rate of virtually every cell in the body.

Their production is controlled by thyroid-stimulating hormone (TSH) from the anterior pituitary, which in turn is regulated by thyrotropin-releasing hormone (TRH) from the hypothalamus.

This hypothalamic-pituitary-thyroid axis operates as a negative feedback loop: when circulating thyroid hormone levels fall, TSH rises to stimulate the thyroid to produce more.

In primary hypothyroidism (the most common type), the thyroid gland itself fails, causing T4 to fall and TSH to rise.

Hypothyroidism is diagnosed by blood tests. The characteristic pattern in primary hypothyroidism is an elevated TSH with a low free T4.

In subclinical hypothyroidism, TSH is elevated but free T4 remains within the normal range.

NICE CKS and the BNF recommend treating overt hypothyroidism (elevated TSH plus low free T4) with levothyroxine.

Treatment of subclinical hypothyroidism is guided by the TSH level, symptom burden, and individual factors such as age, cardiovascular risk, and thyroid antibody status.

Treatment is generally recommended when TSH is above 10 mU/L, and considered when TSH is between 4 and 10 mU/L with symptoms.

How levothyroxine works: mechanism of action

Levothyroxine is a synthetic form of the T4 hormone that is identical in structure to the thyroxine produced by the human thyroid gland.

After oral administration, it is absorbed primarily in the jejunum and ileum.

Absorption is approximately 70 to 80 percent under fasting conditions but is significantly reduced by food, certain medications, and gastrointestinal conditions.

Levothyroxine has a long half-life of approximately 7 days in euthyroid individuals (longer in hypothyroidism, shorter in hyperthyroidism), which allows once-daily dosing and produces stable circulating hormone levels.

Once in the bloodstream, levothyroxine (T4) is converted to the more biologically active triiodothyronine (T3) by deiodinase enzymes in peripheral tissues, particularly the liver, kidneys, and brain.

T3 enters cells and binds to nuclear thyroid hormone receptors, modulating the expression of genes involved in energy metabolism, protein synthesis, thermogenesis, cardiac contractility, cholesterol clearance, bone turnover, and neurodevelopment.

By restoring T4 and T3 levels to the normal range, levothyroxine reverses the metabolic consequences of hypothyroidism.

The goal of treatment is biochemical and clinical euthyroidism: a TSH within the normal reference range (typically 0.4 to 4.0 mU/L) accompanied by resolution of hypothyroid symptoms.

Because T4 to T3 conversion is an endogenous process, levothyroxine monotherapy effectively normalises both T4 and T3 levels in the vast majority of patients.

A small proportion of patients report persistent symptoms despite a normal TSH, which may relate to polymorphisms in deiodinase genes, non-thyroidal illness, or coexistent conditions.

The addition of liothyronine (synthetic T3) remains controversial and is not routinely recommended by NICE or the BNF, though the BTA/SfE position statement acknowledges that a trial of combination therapy may be considered in selected patients who remain symptomatic despite optimised levothyroxine therapy.

Clinical evidence and UK prescribing guidance

Levothyroxine replacement has been the standard of care for hypothyroidism for over 50 years.

The evidence base is extensive, with large observational studies and clinical experience confirming that properly dosed levothyroxine is effective, safe, and well tolerated.

NICE CKS (Clinical Knowledge Summaries) for hypothyroidism recommends levothyroxine as the first-line treatment, with dose titration guided by TSH. The BNF provides detailed dosing guidance and interactions.

The British Thyroid Foundation and the British Thyroid Association provide patient and professional resources respectively.

The MHRA has issued specific guidance on levothyroxine prescribing.

Because different brands of levothyroxine tablets may have slightly different bioavailability (due to differences in excipients and manufacturing processes), the MHRA recommends that levothyroxine should be prescribed by brand name and that patients should remain on the same brand wherever possible.

If a brand change is unavoidable, TSH should be rechecked after 6 to 8 weeks.

The main brands available in the UK include Eltroxin, Teva levothyroxine, Accord levothyroxine, and Wockhardt levothyroxine, as well as unbranded generics.

An oral solution (Eltroxin oral solution) is available for patients who cannot swallow tablets.

Dosage and administration

Take levothyroxine once daily in the morning, on an empty stomach, at least 30 to 60 minutes before breakfast, tea, or coffee.

Swallow the tablet whole with a full glass of water. The starting dose depends on your age, weight, cardiac history, and the severity of hypothyroidism.

Most adults without cardiac disease start at 50 to 100 micrograms daily.

Older patients and those with heart disease start at 25 micrograms daily, increasing by 25 micrograms every 4 to 6 weeks.

The average maintenance dose is 100 to 150 micrograms daily.

Do not take levothyroxine at the same time as calcium supplements, iron tablets, antacids, sucralfate, or cholestyramine; these bind to levothyroxine and prevent absorption.

Allow at least 4 hours between doses. Coffee can also reduce absorption and should be taken at least 30 minutes after levothyroxine.

Blood tests (TSH, and usually free T4) should be checked 6 to 8 weeks after starting or changing the dose. Once stable, annual monitoring is sufficient.

In pregnancy, TSH should be checked every 4 weeks during the first trimester and the dose adjusted promptly if TSH rises above the trimester-specific reference range.

Side effects of levothyroxine

Over-replacement symptoms

Palpitations, tremor, anxiety, insomnia, sweating, heat intolerance, diarrhoea, and weight loss indicate the dose is too high.

Long-term over-replacement risks include atrial fibrillation (especially in the elderly) and osteoporosis (especially in postmenopausal women). If you experience these symptoms, contact your GP for a blood test.

Under-replacement symptoms

Persistent fatigue, weight gain, cold sensitivity, constipation, dry skin, and depression suggest the dose is too low. Your prescriber should recheck TSH and increase the dose if indicated.

Other considerations

True allergic reactions to levothyroxine are extremely rare. Reactions to excipients (lactose, acacia, colourings) in certain brands may occur; switching brands may resolve this.

Transient hair loss in the first few months of treatment, particularly in children, usually resolves spontaneously.

Angina may be provoked in patients with coronary artery disease if the dose is increased too quickly.

When to seek urgent medical advice

Contact your GP or call NHS 111 if you develop persistent palpitations, tremor, chest tightness, or significant mood changes.

Call 999 or attend A&E if you experience chest pain, severe breathlessness, collapse, or signs suggestive of myxoedema coma (extreme drowsiness, hypothermia, confusion in a patient with known untreated or undertreated hypothyroidism).

Report suspected adverse reactions to the MHRA at yellowcard.mhra.gov.uk .

Warnings and precautions

Cardiac disease

In patients with ischaemic heart disease, heart failure, or arrhythmias, start at 25 micrograms daily and increase gradually. Thyroid hormones increase cardiac oxygen demand, and rapid correction of hypothyroidism can precipitate angina, myocardial infarction, or arrhythmias.

Adrenal insufficiency

Patients with coexisting adrenal insufficiency (primary or secondary) must receive adequate glucocorticoid replacement before starting levothyroxine. Without cortisol cover, levothyroxine can trigger an adrenal crisis.

Drug interactions

Calcium carbonate, iron, aluminium antacids, sucralfate, and cholestyramine reduce absorption; separate by 4 hours. Enzyme-inducing drugs (carbamazepine, phenytoin, rifampicin) increase levothyroxine metabolism.

Amiodarone and lithium independently affect thyroid function. Warfarin effect is enhanced by levothyroxine; monitor INR closely. Oestrogen therapy increases TBG, potentially requiring dose increase.

Biotin supplements interfere with thyroid blood test assays and should be stopped 48 hours before testing.

Pregnancy and breastfeeding

Levothyroxine is safe and essential in pregnancy. The dose usually needs to increase by 25 to 50 percent.

Monitor TSH every 4 weeks in the first half of pregnancy. After delivery, return to the pre-pregnancy dose and recheck TSH at 6 weeks postpartum.

Levothyroxine is safe during breastfeeding.

How to get levothyroxine in the UK

Levothyroxine is a prescription-only medicine available through the NHS.

Your GP or an authorised online prescriber registered with the General Pharmaceutical Council (GPhC) can prescribe it after confirming hypothyroidism with blood tests.

The standard NHS prescription charge in England is currently 9.90 pounds per item; however, patients with hypothyroidism are entitled to apply for a medical exemption certificate (via form FP92A), which provides free prescriptions for all conditions, not just thyroid-related ones.

Prescriptions are free in Scotland, Wales, and Northern Ireland regardless of medical condition.

Living well with hypothyroidism

Most people with hypothyroidism who take their levothyroxine consistently feel completely well and live normal, active lives.

Take your medication at the same time every day, keep your annual blood test appointments, and inform your prescriber if you start or stop any new medicines or supplements.

Maintain a balanced diet including adequate iodine (found in dairy products, fish, and seaweed in moderate amounts).

There is no need for special dietary restrictions, though excessive soya and very high-fibre diets may modestly reduce levothyroxine absorption.

Exercise regularly, as physical activity benefits metabolism, mood, and cardiovascular health.

Support organisations such as the British Thyroid Foundation provide reliable information and peer support for patients with thyroid conditions.

Sources

• Levothyroxine Sodium Tablets, Summary of Product Characteristics (EMC)
• Levothyroxine sodium, British National Formulary (BNF)
• NICE CKS: Hypothyroidism
• Levothyroxine, NHS medicines information
• Underactive thyroid (hypothyroidism), NHS
• British Thyroid Foundation
• MHRA Yellow Card Scheme