what causes an underactive thyroid
An underactive thyroid (hypothyroidism) usually happens because the thyroid gland is damaged, inflamed, or not being properly controlled by the brain, so it cannot make enough thyroid hormone. The most common cause in many countries is an autoimmune disease called Hashimoto’s thyroiditis, where the immune system attacks the thyroid.
Main medical causes
- Autoimmune disease (Hashimoto’s) : The body makes antibodies that slowly damage the thyroid, so hormone production drops over time; this is the leading cause in areas with enough iodine in the diet. It is especially common in women and often runs in families.
- Thyroiditis (inflammation) : Infections, autoimmune conditions, or after pregnancy can inflame the gland, sometimes causing a brief overactive phase followed by underactivity once stored hormone is used up. This form can be temporary or, in some people, become long‑term.
- Thyroid surgery or radioactive iodine : Removing part or all of the thyroid, or treating an overactive thyroid or thyroid cancer with radioactive iodine, can leave too little functioning thyroid tissue to make enough hormone. This usually causes permanent hypothyroidism that needs lifelong replacement tablets.
Other important triggers
- Radiation to the neck : Radiotherapy for cancers in the head and neck can injure thyroid cells and gradually lead to an underactive thyroid. People who had childhood neck radiation are at higher long‑term risk and often need periodic blood tests.
- Medications : Drugs like lithium (for bipolar disorder), amiodarone (for heart rhythm problems), interferon‑alpha, and some cancer therapies can reduce thyroid hormone production or trigger thyroiditis. Sometimes the effect reverses when the medicine is stopped, but in other cases hypothyroidism persists.
- Iodine imbalance : The thyroid needs iodine to make hormone, so severe iodine deficiency (more common in some developing regions) can cause hypothyroidism. Too much iodine, from some supplements or contrast dyes, can also push a vulnerable thyroid into being underactive.
Less common causes
- Congenital hypothyroidism : Some babies are born with a missing, small, or abnormally formed thyroid, so they start life with low hormone levels. Newborn screening in many countries picks this up early so treatment can start quickly.
- Pituitary or hypothalamus problems : If the pituitary gland or hypothalamus in the brain is damaged by a tumor, surgery, trauma, or disease, it may not send enough TSH (the “signal” hormone) to the thyroid, causing secondary hypothyroidism. In this case the thyroid itself is normal, but it is not being instructed to work.
- Post‑pregnancy (postpartum thyroiditis) : Some women develop temporary thyroiditis after giving birth, which can progress from overactive to underactive thyroid before often resolving. A proportion of these women go on to develop permanent hypothyroidism later in life.
Risk factors and when to check
- Being female, older than about 60, or having a family history of thyroid or other autoimmune disease all raise the risk of an underactive thyroid. Having type 1 diabetes, celiac disease, or other autoimmune disorders also increases the chance.
- Prior neck radiation, thyroid surgery, or long‑term use of thyroid‑affecting medicines are reasons many doctors recommend periodic thyroid blood tests (TSH, free T4). If someone has symptoms like fatigue, weight gain, feeling cold, dry skin, or low mood, testing is usually straightforward and highly accurate.
TL;DR: Most cases of underactive thyroid are caused by autoimmune damage (Hashimoto’s), thyroiditis, medical treatments (surgery, radiation, certain drugs), iodine problems, or rarer brain–pituitary disorders.
Information gathered from public forums or data available on the internet and portrayed here.