Hypothyroidism (Underactive Thyroid): Symptoms, Diagnosis & NHS Treatment

Hypothyroidism, or underactive thyroid, is a condition in which the thyroid gland does not produce enough thyroid hormone. It is very common in the UK — affecting around 1 in 50 women and 1 in 500 men. Left untreated, it can cause significant health problems, but it is easily managed with a once-daily tablet on the NHS.

What Is the Thyroid Gland?

The thyroid is a small, butterfly-shaped gland at the front of the neck. It produces two main hormones: thyroxine (T4) and triiodothyronine (T3). These hormones regulate metabolism throughout the body — influencing energy, temperature regulation, heart rate, weight, mood, and the function of virtually every organ system.

The thyroid is regulated by the pituitary gland via thyroid stimulating hormone (TSH). When thyroid hormone levels fall, the pituitary increases TSH output to stimulate the thyroid to produce more. In hypothyroidism, the thyroid cannot produce enough hormone to meet the body’s needs — metabolism slows down across the board.

Causes of Hypothyroidism

The most common causes of hypothyroidism in the UK include:

• Hashimoto’s thyroiditis (autoimmune hypothyroidism) — the most common cause. The immune system produces antibodies (anti-thyroid peroxidase, anti-TPO) that attack and gradually damage the thyroid gland. It is more common in women and often runs in families. Associated with other autoimmune conditions including type 1 diabetes and coeliac disease
• Previous treatment for hyperthyroidism — radioiodine (I-131) therapy and thyroidectomy for overactive thyroid commonly result in permanent hypothyroidism requiring lifelong levothyroxine
• Thyroid surgery — for thyroid cancer, goitre, or nodules
• Medications — amiodarone (for heart arrhythmias) and lithium can impair thyroid function; interferon and immunotherapy can trigger autoimmune thyroiditis
• Iodine deficiency — rare in the UK but the commonest cause globally; iodine is required for thyroid hormone synthesis
• Postpartum thyroiditis — occurs in around 5% of women in the year after delivery; often transient, but can be permanent
• Congenital hypothyroidism — detected by newborn blood spot screening; treated from birth

Symptoms of Hypothyroidism

Because thyroid hormone affects every system in the body, symptoms are wide-ranging and non-specific. They develop gradually and are often mistakenly attributed to ageing, depression, or menopause. Common symptoms include:

• Fatigue and low energy — often profound; the most common presenting complaint
• Weight gain despite no change in diet
• Feeling cold — low metabolic rate reduces heat production
• Depression and low mood
• Slowed thinking and poor concentration (“brain fog”)
• Dry skin and hair; hair loss; brittle nails
• Constipation
• Muscle aches, weakness, and cramps
• Slow heart rate (bradycardia)
• Puffy face (periorbital oedema) and swollen ankles
• Hoarse voice
• Irregular or heavy periods in women; reduced libido and fertility problems
• Raised cholesterol — hypothyroidism reduces LDL clearance

How Is Hypothyroidism Diagnosed?

The primary test is serum TSH. A raised TSH indicates the pituitary is working harder to stimulate an underperforming thyroid. TSH is highly sensitive and is the best single screening and monitoring test for thyroid dysfunction.

The UK reference range for TSH is typically 0.4–4.0 mU/L, though this varies slightly between laboratories. Serum free T4 (fT4) is measured alongside TSH to confirm and grade the hypothyroidism:

• Overt (clinical) hypothyroidism — raised TSH with low fT4. Clear indication for treatment
• Subclinical hypothyroidism — raised TSH with normal fT4. NICE advises treatment if TSH is above 10 mU/L, or in symptomatic individuals with TSH 4-10 mU/L

Additional tests include:

• Anti-TPO antibodies — confirms Hashimoto’s thyroiditis; guides prognosis (positive antibodies predict progression)
• Lipid profile — raised cholesterol is common with hypothyroidism
• Full blood count — macrocytic anaemia can occur (vitamin B12 deficiency is associated with Hashimoto’s)
• Vitamin B12 and coeliac serology — associated autoimmune conditions to consider

NHS Treatment: Levothyroxine

The standard treatment for hypothyroidism is daily oral levothyroxine (synthetic T4), branded as Eltroxin in some formulations. It replaces the missing thyroid hormone and, in most people, completely resolves symptoms.

Key prescribing points per NICE NG145:

• Start at a low dose (typically 25-50 micrograms daily) and titrate up every 4-6 weeks until TSH is within the normal range
• In older people and those with cardiovascular disease, start at 25 micrograms due to the risk of precipitating angina or arrhythmia
• The average maintenance dose is 100-150 micrograms daily; doses up to 200 micrograms may be needed
• Take on an empty stomach, 30-60 minutes before breakfast, with water only — food, coffee, and certain supplements reduce absorption
• TSH should be rechecked 6-8 weeks after each dose change; once stable, annually
• Most patients are treated in primary care; refer to endocrinology if dose requirements are unexpectedly high, TSH remains abnormal despite apparent adherence, or if considering combination T3/T4 therapy

Target TSH Range

For most patients on levothyroxine, the target TSH is the lower half of the normal reference range (0.4–2.5 mU/L). Over-replacement (TSH below 0.4 mU/L) increases risks of atrial fibrillation, osteoporosis, and cardiac complications and should be avoided.

Liothyronine (T3) Combination Therapy

A small proportion of patients on levothyroxine continue to have symptoms despite a normal TSH. NICE acknowledges this and suggests referring to endocrinology for consideration of combination T4/T3 therapy (adding liothyronine). This remains controversial — evidence for improved outcomes over T4-only therapy is limited, and liothyronine is significantly more expensive. NICE does not routinely recommend it but acknowledges individual patient variability.

Subclinical Hypothyroidism

Subclinical hypothyroidism (raised TSH with normal fT4) is found in around 5-10% of the population. Most people are asymptomatic. NICE recommends:

• If TSH is 4-10 mU/L and the patient is symptomatic: offer a 6-month trial of levothyroxine and review benefit
• If TSH is 4-10 mU/L and the patient is asymptomatic: monitor annually, as many cases resolve spontaneously
• If TSH is above 10 mU/L: treat with levothyroxine regardless of symptoms, as the risk of progression to overt hypothyroidism and cardiovascular events is higher

Living With Hypothyroidism

• Take levothyroxine consistently — always at the same time of day on an empty stomach; do not skip doses
• Be aware of interactions — calcium, iron tablets, antacids, and proton pump inhibitors all reduce levothyroxine absorption; take them at least 4 hours apart
• Don’t switch brands without discussing with your GP — different formulations can have slightly different bioavailability; consistency matters
• Attend annual blood tests — TSH should be checked at least once a year when stable
• Report new symptoms — if you remain symptomatic on treatment, this should be investigated, not dismissed
• Screen for associated conditions — ask about coeliac screening and vitamin B12 if you have Hashimoto’s