Overview
Hypothyroidism means the thyroid gland does not produce enough thyroid hormone, slowing down metabolism and body functions. The most common cause is Hashimoto thyroiditis, an autoimmune condition. It is highly treatable with daily thyroxine tablets, which usually restore normal function completely.
How common is it?
Hypothyroidism is common and can affect people of any age. It is particularly common in women. Some people have mildly raised TSH with normal thyroid hormone levels, called subclinical hypothyroidism.
Causes and risk factors
The thyroid fails to produce sufficient hormone due to autoimmune destruction, prior treatment, or insufficient iodine.
Common risk factors
- Hashimoto thyroiditis (autoimmune lymphocytic destruction)
- Previous radioiodine treatment or thyroid surgery
- Iodine deficiency (globally the most common cause)
- Medications: amiodarone, lithium, interferon-alpha
- Congenital hypothyroidism
- Pituitary or hypothalamic disease reducing TSH production (central hypothyroidism)
- Post-partum thyroiditis
Symptoms
- Fatigue and low energy
- Weight gain despite unchanged diet
- Feeling cold when others are comfortable
- Constipation
- Dry skin and hair
- Hair thinning or loss
- Depression and slowed thinking (brain fog)
- Muscle aches and cramps
- Heavy or irregular menstrual periods
- Raised blood cholesterol
- Bradycardia (slow heart rate)
- Puffy face and swollen ankles
When to see a doctor
See a doctor if you notice unexplained fatigue, weight gain, or coldness. Myxoedema coma, a rare complication of untreated severe hypothyroidism, is a medical emergency with altered consciousness and hypothermia.
Diagnosis
Blood tests usually measure TSH and free T4. In overt primary hypothyroidism, TSH is high and free T4 is low; subclinical disease has normal free T4. TPO antibodies can support an autoimmune cause. Suspected pituitary disease needs different interpretation.
Treatments
Levothyroxine (L-T4) replacement
Synthetic thyroxine taken once daily, ideally in the morning on an empty stomach. Dose is titrated by TSH level, typically aiming for TSH within the normal reference range. Most patients feel well on this alone.
Combined T4/T3 therapy
Levothyroxine is the standard treatment. NICE does not recommend routine liothyronine or natural thyroid extract. Persistent symptoms need reassessment; any consideration of combined T4/T3 treatment should be led by a thyroid specialist.
Monitoring
TSH should be checked 6 to 8 weeks after starting or changing levothyroxine dose, then annually once stable. Separate calcium supplements, iron, and antacids from levothyroxine by at least 4 hours as they impair absorption.
Self-care and lifestyle
- Take levothyroxine at the same time every day, on an empty stomach
- Avoid taking iron, calcium, or antacids within 4 hours of levothyroxine
- Ensure adequate dietary iodine (dairy, seafood, iodised salt)
- Regular physical activity counteracts fatigue and weight gain
Prevention
Most autoimmune hypothyroidism cannot be prevented. Adequate dietary iodine is important, but do not start iodine or kelp supplements without advice. Newborn screening can detect congenital hypothyroidism early.
Sources for targeted updates
Updated: 24 October 2026. Specific treatment, safety or key-fact statements corrected in this update. This update covers the named sections; a full review of the remaining guide is pending.