Hypothyroidism

Hypothyroidism, or an underactive thyroid, means the thyroid gland produces too little thyroxine (T4) and triiodothyronine (T3), the hormones that set the body's metabolic rate, so many processes slow down. The pituitary gland responds by releasing more thyroid-stimulating hormone (TSH) to push the failing gland harder, and this rise in TSH is the most sensitive sign of the condition.

Across the world, a diet short of iodine, which the gland needs to build its hormones, remains a major cause and can produce a goiter, an enlarged thyroid. Where salt is iodized, the usual cause is Hashimoto's disease, an autoimmune condition in which the immune system slowly destroys thyroid tissue. Thyroiditis, removal of the gland by surgery, radioactive iodine treatment, some medicines and disorders of the pituitary are other causes. It mostly affects women and older people. Symptoms creep on and are easy to miss: tiredness, feeling cold, weight gain, constipation, dry skin, thinning hair, a slow pulse, low mood and heavy or irregular periods.

Some babies are born with an underactive thyroid, which harms growth and brain development unless treated early, so newborns are screened for it. Diagnosis rests on a high TSH with a low T4 in the blood, and treatment is a daily synthetic T4 tablet, levothyroxine, adjusted by follow-up blood tests.