Treatment with thyroid drugs

It should be made clear from the outset, that – whatever the cause of hypothyroidism and hyperthyroidism – thyroid drugs will not cure the underlying condition.

Thyroid drugs, such as levothyroxine, are so-called thyroid hormone replacement medicines; they replace the hormones that the thyroid gland would be making if it was healthy. In case of treatment of thyroid cancer patients, these drugs are also prescribed to stop the growth of healthy thyroid cells and residual thyroid cancer cells that remain in the body after thyroid surgery (thyroid hormone suppression therapy).

Anti-thyroid drugs, such as carbimazole and propylthiouracil, reduce the amount of thyroid hormones produced by the thyroid. In New Zealand, they are usually the first choice of treatment for patients with Graves' disease; they may also be prescribed to patients with toxic nodular or multinodular goitre to prepare them for RAI or surgery.

The ultimate goal of treatment with drugs is to achieve a hormone level that is as close to that produced by a normal, healthy thyroid as possible ("euthyroidism") and, obviously, relief of symptoms. Whether euthyroidism is achieved and maintained should be monitored by means of blood tests. Patients treated for hypothyroidism will usually only have their TSH checked. The monitoring of patients treated for hyperthyroidism involves testing the levels of Free T4, TSH, and especially Free T3. (See our page Diagnosis and monitoring: blood tests for more information about these tests.)

Many patients treated for hypothyroidism will have to take their thyroid hormone replacement drugs for the rest of their life. In the case of hyperthyroidism/ Graves' disease, there is a chance of remission, in which case treatment with carbimazole can be discontinued (after about 12-18 months of treatment), although not always permanently. If remission cannot be achieved, other treatment options (RAI or surgery) may be considered. For more information about RAI and thyroid surgery, see our page Treatment options other than drugs.

Approved and funded brands of levothyroxine

• Synthroid

• Levothyroxine (generic; same formulation, from the same manufacturer, as the Goldshield levothyroxine that was temporarily available in 2008)

• Eltroxin (new formulation 2008 - made in Germany - has led to many reports of serious side effects around the world, including in New Zealand)

Approved but not funded brand of levothyroxine

• Eutroxsig (generic version of the brand Oroxine)

Not approved or funded brands of levothyroxine

• Eltroxin (formulation approved and funded in NZ before 2008 - made in Canada)

• Levothyroxine (Accord generic, rebranded from: Actavis, also supplied as Almus; formulation without acacia)

Not approved or funded brand of liothyronine

• Liothyronine Sodium (generic)

Not approved or funded: desiccated porcine thyroid extract

• Whole Thyroid (capsules)

For more detailed information about these medicines, see our Thyroid drugs information page.

Approved and funded brand of carbimazole

• Neo-Mercazole

Not approved, but funded under Special Authority

• Propylthiouracil (PTU)

For more detailed information about these medicines, see our Thyroid drugs information page.

Side effects - or adverse reactions - can be defined as "undesirable effects to medicines or vaccines". The possible side effects of thyroid drugs are usually well described in the data sheets for these medicines and known to doctors.

As Medsafe explains on its website:

"Whilst some reactions may occur when a medicine is used at higher than recommended doses, adverse reactions may also occur when normal doses are used.

"Reactions may be evident within minutes or years after exposure to the medicine ... 

In addition, we would like to encourage patients and - especially - doctors to also be aware of possible side effects that are less common and not described in the data sheets, particularly if the formulation of a medicine has changed or a brand is being supplied that wasn't previously available.

Many thyroid patients have learned the hard way how debilitating side effects can be after the change in formulation of Eltroxin levothyroxine in 2008, which were initially dismissed by Medsafe and (as a consequence, also) doctors with comments that the effects experienced by patients were the result of poor patient compliance or patients' reactions to media exposure. If patients, and in many other cases also doctors, had not become aware that their reactions were the result of their changed medicine and had not reported their reactions to CARM (see below), we probably would still be in the situation that only one brand of levothyroxine was subsidised (the one causing the many side effects!) instead of the three funded brands we have now.

For the optimal treatment and wellbeing of patients, we need everyone – patients, doctors and pharmacists – to be aware of, and respond to:

• changes in drug formulations and brands of medicines;

• adverse reactions to these medicines;

• the need to report these adverse reactions.

Obviously, we also recommend reporting side effects if a patient has adverse reactions to a drug that isn't new or hasn't changed, but is prescribed to this patient for the first time. We suspect (but can't prove) that in this situation side effects are often not reported, because a patient is simply put on another brand of the drug (if available) or another type of medication.

For completeness, it should also be noted that if a patient doesn't respond well to treatment with a particular medicine, this isn't always because there is something wrong with the medicine. Examples of a few other possible causes of negative response (or lack of positive response) to treatment with a drug are: the medical condition for which the patient is being treated; other medical conditions (diagnosed or non-diagnosed) for which the patient is or is not being treated; other drugs – including over-the -counter medicines, food supplements and herbal products – taken by the patient.

Reporting side effects

We recommend reporting side effects (adverse reactions) of drugs to the Centre for Adverse Reactions Monitoring (CARM) in Dunedin, either by a patient's doctor or by the patient her or himself. It is preferable to use CARM's reporting form, but it is also possible to send an e-mail to CARM as long as all the information required by the form is included in the e-mail. The more complete and detailed the information, the better.

More information:

• How Does Medsafe Monitor Medicine Safety? (Medsafe, last updated 21 November 2023)

• How to Report a Problem (Medsafe, 28 August 2023)

• Reporting an adverse drug reaction (Centre for Adverse Reactions Monitoring, CARM)

Taking too much anti-thyroid drugs such as carbimazole and propylthiouracil (prescribed to treat hyperthyroidism, see above) can cause symptoms of hypothyroidism, just like taking too much thyroid hormone replacement drugs such as levothyroxine or liothyronine (prescribed to treat hypothyroidism) can result in symptoms of hyperthyroidism (thyrotoxicosis).

Some other drugs – including over-the-counter medicines, food supplements and herbal products – can interact with thyroid medicines (levothyroxine, liothyronine), for example,* by:

• decreasing the efficacy of levothyroxine and liothyronine by delaying or preventing their absorption, which may result in hypothyroidism.
  Examples: certain antacids; calcium carbonate; iron; Orlistat (anti-obesity drug); cholesterol lowering drugs, such as colestipol (Colestid).

• increasing the metabolism (breakdown) by the liver of thyroid hormones (irrespective of whether they are made by the thyroid gland or administered as thyroid replacement medication). This may result in hypothyroidism.
  Examples: antiepileptic drugs, such as Carbamazepine (also prescribed for bipolar disorder), Phenobarbital.

• decreasing the conversion of T4 to T3, leading to decreased T3 levels (irrespective of whether the T4 is made by the thyroid gland or administered as thyroid replacement medication).
  Examples: amiodarone, beta-blockers such as Propanolol; glucocorticoids.

Inversely, thyroid drugs can also have an effect on the metabolism, efficacy and toxicity of other drugs (including side effects).

A few examples* of drugs that may be affected by the use of levothyroxine and liothyronine are: oral anticoagulants (e.g., warfarin); heart medications like Digoxin (digitalis glycosides); bronchodilators such as theophylline; certain antidepressants; insulin and other antidiabetic drugs; oestrogen-containing oral contraceptives (birth control pills); and growth hormones.

Similarly, the efficacy of anti-thyroid drugs like carbimazole and propylthiouracil may also interact with other drugs. Examples are iodide, anticoagulants (e.g. Warfarin, heparin), heart medications (digitalis glycosides; beta-blockers); and theophylline.

* As noted, only examples are provided and completeness is not intended.

If your doctor prescribes thyroid drugs to treat your thyroid condition, let them know if you are using other medicines, food supplements and/or herbal products. In case of doubt about possible interactions, ask your doctor or pharmacist. Reading the data sheet of the prescribed thyroid medicine is also recommended.

Also be aware that the extent and consequences of drug (and food) interactions may be patient specific and may be different from patient to patient, depending on factors such as age, sex, race, intercurrent illnesses, dose of either agents, additional concomitant medications, and timing of drug administration.

What to do if you are taking drugs that interfere with your thyroid medication?

Above all, don't stop taking prescription drug(s) without first discussing it with your doctor. If you take drugs that may affect the thyroid hormones levels in your body, your doctor is likely to suggest having regular blood tests to monitor those levels (ask for it, if necessary).

If you are taking medicines that may decrease the absorption of your thyroid medication, don't take them at the same time of the day. Take them at least 4 hours apart!

The same applies if your thyroid medication may interfere with other medicines you are taking. Use a pill organiser if you think this will help you keep track of your medicines.

Not only drugs, but also foods may interfere with the absorption or metabolism of thyroid medication.

Examples of foods or nutrients that may negatively affect the absorption of thyroid medication are foods containing soy; coffee; walnuts and grapefruit (including juice). Certain dietary fibres (e.g., those in bread and other grains) may also impair absorption of thyroid medication. A herbal remedy such as lemon balm – often used in the form of herbal tea or oil – is also known to prevent absorption.

As noted under 'Drug - drug interactions', certain food supplements – particularly, iron and calcium – delay or prevent the absorption of levothyroxine.

Vitamin C, on the other hand, improves the absorption of levothyroxine. Selenium is a nutrient necessary for the conversion of T4 to T3.

What to do if you would like to consume foods that may interfere with the absorption of your thyroid medication?

It is recommended to always take your thyroid medication on an empty stomach.

Take you thyroid drugs:

• at least half an hour before breakfast (one hour is better), if you take your medication in the morning;

• or at least 3 hours after your last meal (longer is better), if you take your medication at bedtime.

Again, food supplements such as iron and calcium should be taken at least 4 hours after (or before) taking your thyroid drugs.