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 affects about 1 to 2% of the UK population, rising to 7 to 10% of women over 60. Subclinical hypothyroidism (mildly elevated TSH with normal thyroid hormones) affects a further 4 to 8%.
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
TSH is the primary screening test; it is elevated in primary hypothyroidism. Free T4 is low. TPO antibodies confirm Hashimoto thyroiditis. Lipid profile often shows elevated total and LDL cholesterol.
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
A minority of patients on adequate levothyroxine remain symptomatic. Adding liothyronine (L-T3) or using a desiccated thyroid extract (T4+T3) may benefit some. Evidence is mixed and use remains debated.
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
Iodine supplementation in deficient populations prevents the most common global cause. Routine neonatal thyroid screening detects congenital hypothyroidism before symptoms develop.