Levothyroxine Tablets

An underactive thyroid, or hypothyroidism, occurs when the thyroid gland does not produce enough thyroid hormone to meet the body's needs, affecting metabolism, energy, and many other body systems. Levothyroxine tablets are a synthetic form of the thyroid hormone thyroxine (T4), used as hormone replacement to restore normal thyroid hormone levels in patients with hypothyroidism. Available in four strengths: 25 mcg, 50 mcg, 75 mcg, and 100 mcg, levothyroxine is one of the most commonly prescribed medicines in the UK and forms the cornerstone of hypothyroid management. A prescription can only be issued following a clinical consultation.

IMPORTANT — NARROW THERAPEUTIC INDEX AND BRAND CONSISTENCY: Levothyroxine has a narrow therapeutic index. Small differences in the amount absorbed can cause significant clinical effects — too much can cause symptoms of an overactive thyroid; too little leaves hypothyroidism undertreated. The MHRA advises that patients should remain on the same brand of levothyroxine once stabilised, as bioavailability may differ between brands and manufacturers. If your brand changes, your prescriber may need to recheck your TSH level. Always inform your prescriber if a different brand has been dispensed.

Levothyroxine tablets contain levothyroxine sodium, a synthetic form of thyroxine, the primary hormone produced by the thyroid gland. Levothyroxine is identical in structure to the T4 hormone produced naturally by the thyroid and acts as direct hormone replacement for patients whose thyroid gland is unable to produce sufficient hormone. It is the first-line treatment for hypothyroidism recommended by NICE and the British Thyroid Association.

Levothyroxine is licensed in the UK for the treatment of hypothyroidism of all causes, including primary hypothyroidism (where the thyroid itself fails), secondary hypothyroidism (resulting from pituitary or hypothalamic disease), and congenital hypothyroidism (from birth). It is also used in the management of thyroid cancer and certain thyroid enlargement conditions, though these indications are typically managed by specialists.

Levothyroxine is available from multiple UK manufacturers under various brand names. Because bioavailability may differ between manufacturers, the MHRA advises that patients remain on the same brand wherever possible once stabilised. Levothyroxine tablets come in several strengths; the four strengths available through this service are 25 mcg, 50 mcg, 75 mcg, and 100 mcg.

Levothyroxine replaces the thyroid hormone T4 that an underactive thyroid cannot produce in sufficient quantities.

T4 to T3 Conversion

After absorption from the gut, levothyroxine enters the bloodstream as T4. Most of the active thyroid hormone in the body is actually T3 (triiodothyronine), which is produced by converting T4 to T3 in peripheral tissues, particularly the liver, kidneys, and muscles, via an enzyme called deiodinase. This conversion of T4 to T3 happens continuously throughout the body and is the primary mechanism through which levothyroxine exerts its effects.

Metabolism Regulation and Thyroid Health

T3, derived from the levothyroxine dose, binds to thyroid hormone receptors in cells throughout the body. Thyroid hormone is essential for the regulation of metabolism, heart rate, body temperature, energy use, and the normal function of virtually every organ system. In patients with an underactive thyroid, insufficient thyroid hormone leads to slowing of metabolic processes, causing fatigue, weight changes, cold intolerance, constipation, depression, and many other symptoms.

Levothyroxine restores thyroid hormone levels to the normal range, supporting thyroid function and relieving the symptoms of hypothyroidism. The dose is titrated to achieve a TSH (thyroid-stimulating hormone) level within the normal reference range. TSH is produced by the pituitary gland and rises when thyroid hormone levels fall, making it the most sensitive marker of thyroid hormone adequacy. T4 hormone therapy with levothyroxine effectively suppresses elevated TSH and restores normal feedback regulation of the thyroid axis.

Always take levothyroxine exactly as your prescriber instructs. The information below summarises key points from the current UK SmPC. Read the Patient Information Leaflet supplied with your medicine before starting treatment.

• Take levothyroxine once daily, in the morning, on an empty stomach, at least 30 minutes before food, drink (other than water), or other medicines.
• Swallow the tablet whole with a full glass of water.
• The empty-stomach instruction is critical. Many foods, drinks, and medicines significantly reduce levothyroxine absorption if taken at the same time. Coffee, milk, and calcium-containing foods are particularly relevant. See the interactions section below.
• Take at the same time every morning to maintain stable hormone levels. Consistency of timing is important.
• Stay on the same brand of levothyroxine wherever possible. If your pharmacy dispenses a different brand or manufacturer, inform your prescriber so your TSH can be rechecked.
• If you miss a dose, take it as soon as you remember on the same morning. If you only remember later in the day, skip that dose and take the next one as normal the following morning. Do not take a double dose to make up for a missed one.
• Do not stop taking levothyroxine without speaking to your prescriber. Hypothyroid management requires ongoing treatment for most patients.
• Your prescriber will arrange blood tests to check your TSH and free T4 levels, typically 6 to 8 weeks after starting or changing the dose. Once the dose is stable, monitoring is usually annual. Attend all monitoring appointments, as the narrow therapeutic index of levothyroxine means small dose differences can have significant clinical effects.

The appropriate levothyroxine dose is highly individual and depends on age, body weight, the cause and severity of hypothyroidism, cardiac status, and other factors. All dose decisions are made by the prescriber and guided by TSH monitoring.

Levothyroxine 25 mcg Tablets

The 25 mcg tablet is typically used as a starting dose in older patients, those with ischaemic heart disease or cardiac risk factors, and patients with severe or longstanding hypothyroidism where a cautious initiation is clinically appropriate. Starting at a low dose and increasing gradually reduces the risk of provoking cardiac symptoms. It is also used for fine-tuning the dose when small adjustments are required.

Levothyroxine 50 mcg Tablets

The 50 mcg tablet is the standard starting dose for most adults under 65 without cardiac risk factors. After 6 to 8 weeks of treatment, TSH is checked and the dose is adjusted if needed. Many patients require doses higher than 50 mcg to achieve a normal TSH.

Levothyroxine 75 mcg Tablets

The 75 mcg tablet allows dosing at an intermediate strength between 50 mcg and 100 mcg. It is used as a maintenance dose for patients stabilised at this level, or as a titration step between 50 mcg and 100 mcg. Using a single 75 mcg tablet is preferable to tablet splitting for patients who require this dose.

Levothyroxine 100 mcg Tablets

The 100 mcg tablet is a commonly used maintenance dose for adults with hypothyroidism. Many adults with primary hypothyroidism require doses in the range of 100 mcg to 150 mcg per day to maintain a normal TSH. Doses above 100 mcg per day may be achieved by combining tablet strengths — for example, 100 mcg plus 25 mcg. All dose adjustments are made by the prescriber based on TSH monitoring results.

When levothyroxine is taken at the correct dose for the individual patient, side effects are uncommon. Most side effects are related to over-replacement, taking more levothyroxine than the body needs, and are symptoms of excess thyroid hormone (thyrotoxicosis). The information below is based on the current UK SmPC.

Signs of Over-Replacement (Too High a Dose)

Contact your prescriber if you develop any of the following, as they may indicate the dose is too high:

• Palpitations, rapid or irregular heartbeat.
• Tremor (shaking hands).
• Feeling hot or sweating excessively.
• Weight loss despite a good appetite.
• Anxiety, nervousness, or irritability.
• Insomnia.
• Diarrhoea.
• Muscle weakness.

Signs of Under-Replacement (Too Low a Dose)

Contact your prescriber if your hypothyroid symptoms return or worsen, as this may indicate the dose needs increasing:

• Fatigue, sluggishness, or feeling cold.
• Weight gain.
• Constipation.
• Low mood or depression.
• Dry skin and hair.

Serious Side Effects: Seek Urgent Medical Attention

Go to your nearest accident and emergency department or call 999 immediately if you experience:

• Chest pain, severe palpitations, or signs of a heart attack. Excess thyroid hormone can increase cardiac workload and may precipitate cardiac events, particularly in older patients or those with pre-existing heart disease.
• Signs of thyroid storm (a rare but life-threatening condition of severe thyroid hormone excess): extremely high heart rate, high temperature, confusion, and agitation. This is rare with oral levothyroxine but requires emergency treatment.
• Signs of a severe allergic reaction such as sudden swelling of the face, lips, tongue, or throat, or difficulty breathing.

Report any suspected side effect to your prescriber, pharmacist, or via the MHRA Yellow Card scheme at yellowcard.mhra.gov.uk.

Levothyroxine Tablets May Be Suitable If You:

• Have a confirmed diagnosis of hypothyroidism of any cause, supported by blood test results showing elevated TSH and/or low free T4.
• Are already prescribed levothyroxine and require a repeat prescription through an online prescribing service.
• Have thyroid cancer or thyroid nodule disease requiring TSH suppression, under specialist guidance.

Levothyroxine Requires Extra Care If You:

• Have ischaemic heart disease, cardiac arrhythmias, or other significant cardiovascular conditions. Levothyroxine increases cardiac workload. Starting at a low dose (25 mcg) and increasing very slowly is essential in these patients.
• Have adrenal insufficiency. Levothyroxine can precipitate an adrenal crisis in untreated adrenal insufficiency by accelerating cortisol metabolism. Adrenal function must be assessed before starting levothyroxine in patients with suspected pituitary or adrenal disease.
• Are pregnant. Thyroid hormone requirements typically increase in pregnancy. Women with hypothyroidism who become pregnant should have their levothyroxine dose reviewed promptly and TSH monitored more frequently throughout pregnancy.
• Are elderly. Older patients are more sensitive to levothyroxine and should be started at 25 mcg with slow titration.
• Have diabetes. Levothyroxine can affect blood glucose control. Blood glucose monitoring may need to be more frequent when starting or adjusting levothyroxine.

Levothyroxine absorption from the gut is significantly affected by many common foods, supplements, and medicines. All of the following must be taken at least 2 hours, preferably 4 hours, after the levothyroxine morning dose:

• Calcium carbonate supplements and calcium-containing antacids: significantly reduce levothyroxine absorption. This is one of the most clinically relevant interactions for patients taking calcium supplements.
• Iron tablets (ferrous sulphate, ferrous fumarate): markedly reduce levothyroxine absorption. Take iron at least 2 hours after levothyroxine.
• Antacids containing aluminium or magnesium: reduce absorption.
• Bile acid sequestrants (colestyramine, Colestid, Cholestagel): substantially reduce levothyroxine absorption. Take levothyroxine at least 4 to 6 hours before any bile acid sequestrant.
• Proton pump inhibitors (omeprazole, lansoprazole): may reduce absorption in some patients.
• Coffee: even black coffee reduces levothyroxine absorption when taken at the same time. Take levothyroxine with water only and wait at least 30 minutes before coffee.
• Soy products and high-fibre foods: may reduce absorption if consumed at the same time as levothyroxine.
• Warfarin: levothyroxine enhances the anticoagulant effect. INR monitoring is required when starting or adjusting levothyroxine in patients taking warfarin.
• Antidiabetic medicines: levothyroxine may affect blood glucose and reduce the effectiveness of antidiabetic medicines. More frequent blood glucose monitoring is needed when doses change.

Take levothyroxine on an empty stomach with water only, at least 30 minutes before any food, drink, or other medicines. This is the single most important instruction for ensuring consistent and effective absorption.

Levothyroxine is a Prescription Only Medicine. The clinical consultation is the essential first step. A prescription will only be issued if a registered prescriber concludes, based on their individual assessment of your circumstances, that levothyroxine is clinically appropriate for you.

During your online consultation with Pharmacy Planet, a registered prescriber will review the information you provide. This includes your thyroid blood test results (TSH and free T4), current dose if already on treatment, current medicines, relevant medical history, and any symptoms suggesting over or under-replacement. The prescriber will confirm the appropriate dose and strength and will check for any absorption-affecting medicines in your current regimen.

Because levothyroxine has a narrow therapeutic index, blood test monitoring is an essential ongoing component of safe hypothyroid management. Pharmacy Planet provides repeat prescriptions for patients already stabilised on a levothyroxine dose, supported by up-to-date thyroid function results. Dose changes should always be followed by a TSH recheck at 6 to 8 weeks.

Not all consultations will result in a prescription. The prescriber may request recent blood test results before prescribing or recommend a dose review by your GP. This process is designed to protect your safety.

Store below 25°C. Store in the original packaging, away from light and moisture. Levothyroxine tablets are sensitive to moisture and light and must not be stored in a bathroom cabinet or near a kitchen sink. Keep out of the sight and reach of children. Do not use after the expiry date printed on the packaging. Do not transfer tablets to a pill organiser unless instructed to do so, as exposure to humidity and light can degrade the medicine. Do not dispose of medicines via household waste or wastewater. Return unused or out of date tablets to your pharmacist for safe disposal. Full storage instructions are provided in the Patient Information Leaflet supplied with your medicine.

Pharmacy Planet is a GPhC-registered online pharmacy providing access to prescription medicines through a clinically led consultation process. Our registered prescribers and pharmacists are committed to patient safety, clinical accuracy, and responsible prescribing. For medicines with a narrow therapeutic index such as levothyroxine, careful prescribing, consistent brand dispensing, and ongoing monitoring are essential. We follow GPhC standards, MHRA guidance, and ASA advertising regulations in all aspects of our service. Every prescription is reviewed and dispensed by qualified healthcare professionals.

If you have questions about levothyroxine or thyroid hormone therapy, our pharmacy team is available to help. We are committed to transparent, patient centred care.

References

1. Electronic Medicines Compendium (eMC). Levothyroxine sodium tablets — Summary of Product Characteristics. https://www.medicines.org.uk/em. Accessed May 2026.
2. NHS. Underactive thyroid (hypothyroidism). https://www.nhs.uk/conditions/underactive-thyroid-hypothyroidism. Accessed May 2026.
3. NHS. Levothyroxine. https://www.nhs.uk/medicines/levothyroxine. Accessed May 2026.
4. Joint Formulary Committee. British National Formulary (BNF). Levothyroxine sodium. BMJ Group and Pharmaceutical Press. https://bnf.nice.org.uk. Accessed May 2026.
5. Medicines and Healthcare products Regulatory Agency (MHRA). Levothyroxine: advice on switching between brands. https://www.gov.uk/drug-safety-update. Accessed May 2026.
6. Medicines and Healthcare products Regulatory Agency (MHRA). Yellow Card — report a side effect. https://yellowcard.mhra.gov.uk. Accessed May 2026.