Hashimoto's thyroiditis and underactive thyroid: symptoms, tests and levothyroxine

The thyroid gland makes hormones that regulate energy use. In Hashimoto's disease, immune cells and antibodies gradually damage the gland. The NHS describes it as the immune system attacking the thyroid so that it cannot produce enough hormone. The British Thyroid Foundation explains that thyroid antibodies of this kind develop when the immune system mistakenly attacks thyroid cells and tissue, leading to inflammation, tissue damage and disrupted function.

Hashimoto's is one cause among several. The NHS also lists 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. Having relatives with a thyroid condition raises the likelihood.

Some people are found to have antibodies without any disease. The British Thyroid Foundation says positive antibodies can be present in people without thyroid disease, so a positive result needs interpreting alongside symptoms and the TSH level.

The NHS says symptoms may be mild and not easy to notice, usually develop slowly and get worse over time. Many are common and have other causes, which is why a blood test is the way to separate thyroid disease from other explanations. NICE also reminds clinicians that in menopausal women thyroid symptoms can be mistaken for menopause.

• Feeling extremely tired and feeling cold more than usual.

• Putting on weight and constipation.

• Difficulty concentrating or thinking clearly, and low mood or depression.

• Dry skin, dry hair or hair loss.

• A croaky or hoarse voice.

• Irregular or heavy periods in people who have them.

A GP asks about symptoms, any family history of thyroid or autoimmune conditions and recent pregnancy, may feel the neck for swelling, and arranges blood tests. The NHS says these check thyroid-stimulating hormone (TSH) and thyroxine (T4).

NICE guideline NG145 advises measuring TSH alone in adults when pituitary disease is not suspected. If TSH is above the reference range, free T4 is measured in the same sample. Testing is not advised during an acute illness unless the illness itself is thought to be due to thyroid dysfunction, because acute illness can alter the results. NICE also asks clinicians to check about biotin supplements, because high biotin intake may give falsely high or low results.

To confirm an autoimmune cause, NICE suggests considering a thyroid peroxidase antibody (TPOAb) test in adults with a raised TSH, but not repeating it. The British Thyroid Foundation agrees that it is rarely useful to repeat TPOAb, as the level does not influence treatment, and notes that TPO antibodies usually stay in the body and hardly ever normalise completely even when hormone levels are restored.

The NHS says an underactive thyroid is usually treated with levothyroxine tablets that replace the missing hormone, usually for life, and that most people should be able to live a normal life. NICE recommends levothyroxine as first-line treatment and advises against routinely offering liothyronine, alone or alongside levothyroxine, or natural thyroid extract, because evidence of added benefit is lacking and long-term adverse effects are uncertain.

Dosing is individual and set by the prescriber. NICE suggests a lower starting dose for adults aged 65 or over and for those with cardiovascular disease, then adjusting gradually. The aim is to keep TSH inside the reference range and to avoid doses that suppress TSH or cause an overactive thyroid.

Taking too much or too little causes problems. The NHS lists feeling or being sick, diarrhoea, headaches and problems sleeping as signs of the wrong dose and suggests talking to a GP or pharmacist about side effects.

Blood tests are the main follow-up. NICE suggests that for adults on levothyroxine, TSH is considered every 3 months until it settles, defined as 2 similar results in range 3 months apart, and then yearly. It also notes that TSH can take up to 6 months to return to the reference range after a very high starting level or a long period untreated, which is taken into account when changing doses.

Where symptoms continue despite a normal TSH, NICE says free T4 can be measured as well. Some people feel that symptoms persist even when blood tests look right, and the guideline accepts that a clinician may adjust the dose to improve wellbeing, within limits that avoid over-treatment.

The NHS says an underactive thyroid is not usually serious with the right treatment. If it is untreated, or the dose is wrong, there is an increased risk of heart problems, including coronary heart disease and heart failure, and of bone problems including osteoporosis. In very rare cases a severe underactive thyroid causes myxoedema coma, a life-threatening emergency that is more common in older people and needs hospital treatment.

In pregnancy, untreated hypothyroidism can raise the chance of pre-eclampsia, birth defects, premature birth or miscarriage, according to the NHS, which says treatment usually helps prevent these and that the levothyroxine dose is likely to change.

The British Thyroid Foundation notes that people with autoimmune thyroid disease are somewhat more likely to develop another autoimmune condition such as Addison's disease, pernicious anaemia or coeliac disease, although the risk is still very small. New or unexplained symptoms are worth mentioning for that reason. The same source states that in approximately 50% of people with positive TPO antibodies and borderline thyroid disease it will progress to overt hypothyroidism over around 20 years.