Levothyroxine for an underactive thyroid: how replacement treatment works

The NHS describes an underactive thyroid as a common condition in which the thyroid gland in the neck does not make enough hormones. Symptoms can include extreme tiredness, feeling cold, weight gain, constipation, difficulty concentrating, low mood, dry skin and hair, hair loss, a hoarse voice and irregular or heavy periods. They usually develop slowly and get worse over time.

In the UK the main cause, according to the NHS, is Hashimoto's disease, an autoimmune condition in which the immune system attacks the thyroid. Other causes include other thyroid conditions such as thyroiditis, surgery or radiotherapy around the neck or thyroid, treatment for an overactive thyroid, very low iodine intake (rare in the UK) and certain medicines.

Because the problem is a shortage of hormone, treatment replaces it. The NHS says this is usually levothyroxine tablets.

A GP will ask about symptoms and about family history of thyroid or autoimmune conditions, and may examine the neck. The NHS says blood tests check how much thyroid-stimulating hormone (TSH) and thyroxine (T4) are in the blood.

NICE says that, in adults without suspected pituitary disease, measuring TSH alone can be considered first; if it is above the reference range, free thyroxine (FT4) is measured in the same sample. NICE also says to ask about biotin supplements, because high biotin intake can lead to falsely high or low results, and to avoid testing during an acute illness unless thyroid disease is suspected as the cause.

NICE says thyroid peroxidase antibodies may be measured in adults with a TSH above the reference range, but not repeated.

NICE recommends levothyroxine as first-line treatment. It says not to routinely offer liothyronine, alone or with levothyroxine, because there is not enough evidence of benefit over levothyroxine and long-term adverse effects are uncertain, and not to offer natural thyroid extract, which does not have a UK marketing authorisation and has uncertain safety.

For adults under 65 with primary hypothyroidism and no history of cardiovascular disease NICE says to consider starting at 1.6 micrograms per kilogram of body weight per day, rounded to the nearest 25 micrograms. For adults aged 65 and over, or with a history of cardiovascular disease, it says to consider a starting dose of 25 to 50 micrograms per day with titration. The NHS says your doctor may prescribe a low dose and increase it gradually to find the right dose.

Tablets or liquid are taken once a day. The NHS says that if you miss a dose you should take it when you remember unless it is nearly time for the next one, and never take two doses together to make up.

NICE says the aim is to maintain TSH within the reference range, and that if symptoms persist the dose may be adjusted further to achieve optimal wellbeing while avoiding doses that suppress TSH or cause thyrotoxicosis. It also notes that TSH can take up to 6 months to return to the reference range for people who had a very high TSH or a long period of untreated hypothyroidism.

For adults taking levothyroxine, NICE says to consider measuring TSH every 3 months until the level has stabilised (two similar measurements within the reference range 3 months apart) and then once a year. FT4 may be measured as well if symptoms continue. The NHS says regular blood tests check you are on the right dose, and that the wrong dose can cause side effects.

NICE says symptoms may lag behind treatment changes by several weeks to months, and that day-to-day changes in unexplained symptoms are unlikely to be due to underlying thyroid disease because the body has a large reservoir of thyroxine.

The NHS says to take levothyroxine on an empty stomach, 30 to 60 minutes before your first meal of the day, any drink with caffeine such as tea or coffee, or any other medicine, at the same time each day, swallowing tablets whole with a glass of water.

Some medicines and supplements may not mix well with it. The NHS lists medicines for epilepsy such as carbamazepine, acid-reducing medicines such as omeprazole, semaglutide, antidepressants such as sertraline, antacids and other indigestion treatments, calcium and iron supplements, some beta blockers and digoxin, St John's wort, and biotin (vitamin B7), which can affect thyroid tests. Tell your doctor about all other medicines, herbal remedies, vitamins or supplements before starting.

Side effects usually happen when the dose is too high, according to the NHS, and can include difficulty sleeping, headaches, anxiety, sweating and hair loss. Rare serious effects include an allergic reaction and thyroid crisis.

The NHS says an underactive thyroid is not usually serious with the right treatment, but if untreated or on the wrong dose there may be an increased risk of heart problems (including coronary heart disease and heart failure) and bone problems such as osteoporosis. In very rare cases a severe underactive thyroid can lead to myxoedema coma, which is a life-threatening emergency more common in older people and needs treatment in hospital.

Call NHS 111 if you are taking levothyroxine and think you might be having serious side effects. The NHS says to speak to a pharmacist as soon as possible if you have taken more than your prescribed dose, and to use 111 if you cannot reach one or took too much on purpose.

The NHS says that in pregnancy the dose is likely to change, and untreated underactive thyroid can raise the chance of serious complications including pre-eclampsia, birth defects, premature birth or miscarriage, though treatment usually helps prevent these.

Hypothyroidism is diagnosed by a GP using blood tests, and the NHS says a GP may also refer you to a specialist for more tests. Levothyroxine is only available with a prescription.

Private thyroid blood tests and endocrinology consultations are available. NICE guidance on diagnosis and treatment is the same, and it advises against treatments such as natural thyroid extract and routine liothyronine because evidence of benefit is lacking.