Hypothyroidism
Underactive thyroid — diagnosis and levothyroxine management
Hypothyroidism is a common endocrine disorder in which the thyroid gland fails to produce sufficient thyroid hormone. It affects around 2% of the UK population and is significantly more common in women. Left untreated, it can cause serious metabolic, cardiovascular and neurological consequences.
Understanding hypothyroidism
The thyroid gland produces thyroxine (T4) and triiodothyronine (T3), which regulate metabolism throughout the body. In primary hypothyroidism — the most common form — the thyroid itself fails, leading to elevated thyroid-stimulating hormone (TSH) as the pituitary gland attempts to stimulate more production. TSH >10 mIU/L on two occasions is the standard diagnostic threshold for initiating treatment per NICE NG145.
Causes
The most common cause in the UK is Hashimoto's thyroiditis — an autoimmune condition in which antibodies (anti-TPO, anti-thyroglobulin) destroy thyroid tissue. Other causes include previous radioiodine therapy or thyroid surgery, iodine deficiency (rare in the UK), lithium or amiodarone therapy, and central (secondary) hypothyroidism from pituitary or hypothalamic disease. Subclinical hypothyroidism (elevated TSH with normal free T4) affects around 10% of women over 60 and is managed on an individual basis.
Symptoms
Symptoms develop insidiously and include fatigue, weight gain, cold intolerance, constipation, dry skin, hair thinning, depression, bradycardia, hoarse voice, and menstrual irregularities. Myxoedema (severe, longstanding hypothyroidism) can cause facial puffiness, non-pitting oedema, and in its most extreme form — myxoedema coma — a life-threatening emergency characterised by hypothermia, impaired consciousness, and bradycardia requiring intensive care treatment with IV liothyronine.
Treatment with levothyroxine
The standard treatment is levothyroxine (synthetic T4) taken orally once daily, 30–60 minutes before food. The typical starting dose is 1.6 mcg/kg/day. In the elderly or those with cardiac disease, start at 25–50 mcg OD and titrate slowly. Target TSH is 0.4–4.0 mIU/L (NICE NG145). Significant drug interactions affect levothyroxine absorption: calcium, iron supplements, PPIs, and cholestyramine all reduce absorption — take levothyroxine at least 4 hours apart from these. Repeat TSH 6–8 weeks after any dose change.
Living with hypothyroidism
Once stable on treatment, TSH monitoring every 12 months is recommended. Most people with well-controlled hypothyroidism live completely normal lives. If symptoms persist despite a normal TSH, consider other causes — do not automatically increase the levothyroxine dose. Some patients request combination T4/T3 therapy (liothyronine); this is not routinely recommended by NICE but may be considered by specialists in refractory cases. Levothyroxine requirements increase during pregnancy — refer urgently to obstetric endocrinology. Always take the same brand of levothyroxine to avoid variability. This information reflects current evaluation of NICE NG145 guidance.