Levothyroxine: uses, dosage and side effects
Levothyroxine is a synthetic thyroid hormone used to replace what an underactive thyroid no longer makes. For most people it is a lifelong daily tablet, adjusted against blood tests. It is prescription-only in Ireland. This page explains what levothyroxine does, how and when to take it, what interferes with absorption, and how a repeat prescription works.
What levothyroxine is used for
Levothyroxine is used to treat an underactive thyroid, or hypothyroidism, whether it follows autoimmune thyroiditis, thyroid surgery, radioactive iodine treatment or another cause. It is identical in action to thyroxine, the hormone the thyroid gland normally produces, and the body converts it to the more active hormone as it needs to. Because it is replacement rather than stimulation, the aim is to restore normal levels rather than to exceed them, and the right dose is the one that puts your thyroid blood tests in the target range set by your doctor. It is also used after thyroid cancer and in some cases of goitre, at doses a specialist sets. Treated properly, hypothyroidism is compatible with an entirely normal life; the symptoms of tiredness, cold intolerance, weight gain, dry skin and low mood generally resolve as levels normalise.
Side effects of levothyroxine
At the correct dose levothyroxine causes few side effects, because it replaces a hormone the body should have. Almost all the reactions listed in the product information reflect too much: palpitations, a fast or irregular heartbeat, tremor, restlessness, difficulty sleeping, sweating, heat intolerance, headache, diarrhoea, muscle weakness and unintended weight loss. Over-treatment sustained over years is associated with atrial fibrillation and with reduced bone density, which is the reason for periodic blood tests rather than a set-and-forget prescription. Too little brings back the symptoms of an underactive thyroid. Allergic reactions to excipients are possible. Contact your doctor if you develop palpitations, chest pain or a persistently racing pulse, and seek urgent help for chest pain or severe breathlessness. Suspected reactions can be reported to the HPRA.
Dosage and how to take levothyroxine
Levothyroxine is taken once a day on an empty stomach, most often in the morning, at least 30 minutes before breakfast and before tea or coffee, with a glass of water. An alternative timing used by some people is at bedtime, several hours after the last food; either is acceptable provided it is consistent, because consistency is what makes the blood tests interpretable. The starting dose depends on your age, your weight, how severe the deficiency is and whether you have heart disease, and it is adjusted in small steps against thyroid function tests taken some weeks after each change, since the level takes time to settle. Do not adjust the dose yourself on the basis of how you feel. If you miss a dose, take it when you remember on the same day; if a whole day is missed, carry on as normal rather than doubling up.
Is levothyroxine prescription-only in Ireland?
Yes. Levothyroxine is prescription-only in Ireland and cannot be bought over the counter, and thyroid preparations sold online as supplements are not a substitute; some have been found to contain undeclared hormone in unpredictable amounts. The reason for prescriber control is that the dose is individual and is set by measurement, not by symptoms: too little leaves you hypothyroid and too much carries real cardiac and bone risks, and the only way to know which side you are on is a blood test. Diagnosis matters too, since tiredness and weight change have many causes and treating a thyroid that is working normally does harm. If you suspect an underactive thyroid, ask your GP for thyroid function tests rather than starting anything.
Continuing levothyroxine and getting a repeat prescription
If you are established on levothyroxine under a GP or endocrinology service in Ireland, your dose is settled and your supply is running low, you can ask a doctor to review a continuation of that treatment through our online medical questionnaire. There is no video consultation and no phone appointment: you complete a structured medical questionnaire, a doctor reviews it, and if continuation is appropriate you receive a prescription document by email as a PDF laid out to the Annex of Directive 2012/52/EU. You bring it to any pharmacy with photo identification. There is no PIN code and no national identity number on it. The assessment for a repeat prescription costs 19,90 €. The fee covers the medical assessment, not the issuing of a document. If the doctor declines on medical grounds, the fee is refunded. If you do not supply the documentation the doctor asks for, the fee is not refunded. The doctor will ask for your current dose and your most recent thyroid function results with their date, because a continuation without knowing where your levels sit is not responsible prescribing. Where your last blood test is out of date, expect to be asked to arrange one with your own GP. Starting treatment or changing a dose belongs with the clinician who can test and follow you.
Cost and reimbursement in Ireland
Levothyroxine is an inexpensive, long-established medicine, but what you pay depends on the strength, the pack and the HSE arrangement covering you. Medical card holders receive prescribed items under the General Medical Services scheme at the charge that applies at the time. Anyone without a card can register with the HSE for the Drugs Payment Scheme, which caps what a person or family pays for approved prescribed items in a calendar month. People with certain long-term conditions may also be covered by the Long Term Illness Scheme, and whether a specific condition and item qualify is set out by the HSE rather than by a pharmacy or a commercial page, so check there or ask your pharmacist. Charges, thresholds and scheme rules change from time to time.
Interactions and alcohol
Levothyroxine absorption is easily disturbed, which is why the timing rules exist. Calcium salts, iron supplements, magnesium and aluminium antacids, sucralfate, cholestyramine and some phosphate binders bind it in the gut, and they are separated from the dose by around four hours. Proton pump inhibitors can reduce absorption by changing stomach acidity. Soya-rich food, coffee taken too soon after the tablet and high-fibre meals also reduce it. In the other direction, enzyme-inducing medicines such as carbamazepine, phenytoin and rifampicin increase how quickly it is cleared. Levothyroxine itself changes the effect of warfarin and of insulin and other diabetes medicines, so those may need adjusting when your thyroid dose changes. Alcohol does not interact directly, so ordinary moderate drinking does not affect treatment.
Who should not take levothyroxine
Levothyroxine is contraindicated in untreated adrenal insufficiency, in untreated thyrotoxicosis and in acute myocardial infarction, acute myocarditis and acute pancarditis. It is introduced with particular caution and at a low starting dose in older people and in anyone with coronary artery disease, angina, heart failure or a history of arrhythmia, because raising the metabolic rate increases the demand on the heart. Care is also needed in long-standing severe hypothyroidism and in diabetes, where control may shift as levels normalise. Anyone with an allergy to an excipient in a particular product, lactose for instance, needs a preparation that avoids it. This is a summary rather than the full list, which is in the leaflet supplied with your product and is a matter for your prescriber.
Generic and brand names
Levothyroxine sodium is the international non-proprietary name of the active substance, and it is the name any European pharmacy recognises. In Ireland it is supplied as tablets in a range of strengths measured in micrograms, under the substance name and under brand names, and an oral solution exists for people who cannot take tablets. Levothyroxine has a narrow therapeutic range, so if your tablets come from a different manufacturer than before, mention it to your pharmacist and tell your doctor, since some people are sensitive to a change and a check of thyroid function after switching is a reasonable precaution. Strengths in micrograms are close together and easy to misread, so check the number on the carton against your prescription every time. Cross-border prescriptions state the substance internationally with the strength.
Pregnancy and breastfeeding
Levothyroxine is not only compatible with pregnancy, it is important in it: thyroid hormone is essential for the developing baby's brain, and requirements typically rise during pregnancy, so the dose usually needs increasing rather than stopping. If you become pregnant or are planning to, contact your GP or endocrinology service promptly to arrange thyroid function testing and closer monitoring through the pregnancy; do not adjust the dose yourself and do not stop the tablet. Untreated hypothyroidism in pregnancy carries risks for both mother and baby. Breastfeeding is not a reason to stop levothyroxine; the amounts in breast milk are small and treatment continues normally. After the birth the dose is usually reviewed with a blood test as requirements return towards their previous level.
Patient information leaflet
The patient information leaflet inside your pack is the authoritative document for the exact levothyroxine product you hold. It gives the licensed indications, the strengths, the timing instructions relative to food and drink, the full side effect list with frequency categories, the long list of medicines and foods that interfere with absorption, and the contraindications. The HPRA publishes leaflets and the fuller summary of product characteristics for medicines authorised in Ireland, and the same documents appear on the Irish medicines information portal. Read the leaflet for your product, especially if the manufacturer has changed, and keep it with the pack. Because this is usually a lifelong treatment, it is worth rereading the timing and interaction sections whenever another medicine or a supplement is added to your list, since a new calcium or iron product is the most common reason for a stable dose to stop working. Follow the leaflet and your prescriber over anything you read online, this page included.
About this page
This page was written by the medical team at MEDINOW Sp. z o.o. and reviewed by lek. Damian Wojno, a doctor registered in Poland, PWZ no. 3211301. He holds a Polish licence to practise; he is not on the register of the Irish Medical Council, and this page does not suggest otherwise. Content verified on 29 August 2026 against the summary of product characteristics and the patient information leaflet published for the Irish market, and against public guidance from the HPRA and the HSE. This page is information, not advice for your particular case, and it is not an advertisement for a prescription medicine. Nothing here replaces an assessment by your own GP or a hospital clinician. If you are unwell and need urgent help, contact your GP, an out-of-hours service, or call 112 or 999.
Is levothyroxine prescription-only in Ireland?
Yes. It cannot be bought over the counter, because the dose is individual and set by blood tests rather than by symptoms. Thyroid products sold online as supplements are not a substitute and some have contained undeclared hormone.
How long does levothyroxine take to work?
Symptoms usually improve over several weeks, and thyroid function tests are normally repeated some weeks after starting or after a dose change, because blood levels take time to settle. Do not raise the dose yourself while waiting.
What are the common side effects of levothyroxine?
At the correct dose, few. Most reactions listed in the product information are signs of too much hormone: palpitations, tremor, sweating, heat intolerance, insomnia, diarrhoea and weight loss. Contact your doctor if those appear, as the dose may need reducing.
Can I drink alcohol while taking levothyroxine?
Alcohol does not interact directly with levothyroxine, so moderate drinking does not affect treatment. Timing matters more: the tablet is taken on an empty stomach, separated from food, coffee, calcium and iron.
How do I get a repeat prescription for levothyroxine?
If you are established on a settled dose under an Irish GP or endocrinology service, you can ask a doctor to review a continuation through our questionnaire for 19,90 €. You will be asked for your dose and your most recent thyroid function results. The fee covers the assessment, not the issuing of a document; it is refunded if the doctor declines on medical grounds and is not refunded if you do not supply the documentation requested.