What causes hypothyroidism in adults and children? 

Hypothyroidism is a clinical state characterized by an insufficient production of thyroid hormones by the thyroid gland. These hormones are vital for maintaining the metabolic rate and supporting the growth and development of almost every tissue in the body. While the condition can affect individuals at any stage of life, the underlying triggers often differ significantly between adults and children. In adults, the cause is frequently related to an acquired autoimmune response or the late effects of medical treatments. In children, the causes may be congenital, meaning they are present from birth, or acquired during the developmental years. Understanding these diverse causes is essential for ensuring that both age groups receive the appropriate diagnostic screening and long term management required to maintain their health. 

What We’ll Discuss in This Article 

• The role of Hashimoto’s disease as the leading cause of adult hypothyroidism. 

• How previous medical interventions, such as surgery and radiation, impact thyroid function. 

• The impact of specific medications on hormone production in adults. 

• Causes of congenital hypothyroidism in newborns and infants. 

• How iodine deficiency and excess affect thyroid health across all ages. 

• The development of acquired hypothyroidism in children and adolescents. 

• Rarer secondary causes involving the pituitary gland. 

Autoimmune thyroiditis is the primary cause of hypothyroidism in adults. 

The most common reason adults in the United Kingdom develop an underactive thyroid is an autoimmune condition known as Hashimoto’s disease. In this disorder, the immune system produces antibodies that mistakenly target and attack the thyroid gland. Over time, this persistent attack leads to chronic inflammation and a gradual loss of the gland’s ability to manufacture thyroxine (T4) and triiodothyronine (T3). According to the NHS, this condition is significantly more common in women than in men and often has a strong genetic component, frequently running in families. 

The progression of Hashimoto’s is typically slow, with symptoms emerging gradually as the thyroid tissue is replaced by fibrous scar tissue. In some patients, the immune system’s activity may cause the gland to swell, resulting in a goitre. While the exact trigger for this autoimmune response is not fully understood, environmental factors and hormonal changes are thought to play a role. Because it is a progressive condition, most adults diagnosed with Hashimoto’s will eventually require lifelong hormone replacement therapy to prevent the systemic effects of a slow metabolism. 

Medical treatments for other conditions often result in adult hypothyroidism. 

A significant number of hypothyroidism cases in adults are “iatrogenic,” meaning they are a direct result of medical treatment. For instance, individuals who have undergone surgery to remove part or all of the thyroid gland (thyroidectomy) for thyroid cancer or a large goitre will lose their natural hormone production capacity. Similarly, radioactive iodine therapy, a common treatment for an overactive thyroid (hyperthyroidism) or Graves’ disease, works by intentionally destroying thyroid cells. This frequently leads to a permanent underactive state months or years after the procedure. 

Furthermore, radiation therapy for cancers in the head and neck region can inadvertently damage the thyroid gland if it is within the treatment field. As noted by Cancer Research UK, patients receiving such treatments are monitored closely with regular blood tests. Because the thyroid is highly sensitive to radiation, the damage may not manifest immediately, necessitating long term surveillance of thyroid function for years following the completion of cancer treatment. 

Certain medications can disrupt the production of thyroid hormones in adults. 

The thyroid gland is sensitive to a variety of pharmaceutical agents that can interfere with its normal functioning. One of the most common medications linked to hypothyroidism is lithium, which is used to manage bipolar disorder. Lithium can block the release of hormones from the thyroid, leading to an underactive state in a subset of patients. Another important medication is amiodarone, used for heart rhythm disorders, which contains high levels of iodine that can paradoxically shut down hormone production in susceptible individuals. 

Other medications, such as certain “checkpoint inhibitors” used in modern immunotherapy for cancer, can trigger an autoimmune inflammation of the thyroid. The NICE guidelines emphasize that healthcare providers should conduct baseline thyroid function tests before starting these medications and continue to monitor the patient throughout their treatment. Identifying drug induced hypothyroidism is crucial because, in some cases, the condition may resolve if the medication can be safely discontinued or adjusted. 

Congenital hypothyroidism is the most common cause in newborns. 

In children, the most frequent cause of an underactive thyroid is congenital hypothyroidism, where the baby is born with a thyroid gland that has not developed properly or is missing entirely. This occurs in approximately 1 in every 2,000 to 3,000 births in the United Kingdom. In most cases, the gland is in the wrong place (ectopic) or is much smaller than it should be (hypoplasia). Less commonly, the gland is present and in the correct location but cannot produce hormones due to a genetic defect in the chemical processes required for hormone synthesis. 

Because thyroid hormones are essential for brain development and physical growth in infants, the UK performs universal newborn screening. The “heel prick” test, usually conducted five days after birth, measures TSH levels to identify affected infants. According to the British Society for Paediatric Endocrinology and Diabetes, early detection and immediate treatment with levothyroxine are vital to prevent permanent developmental delays and ensure the child reaches their full growth potential. 

Acquired hypothyroidism in children is often due to Hashimoto’s disease. 

While babies are often born with the condition, older children and adolescents can also develop hypothyroidism later in childhood. This is known as acquired hypothyroidism. Just like in adults, the most common cause in this age group is Hashimoto’s thyroiditis. It often becomes apparent during puberty when the body’s demand for thyroid hormones increases. Parents may notice that the child is growing more slowly than their peers, experiencing a decline in school performance, or appearing unusually tired and sluggish. 

In children, the signs of an underactive thyroid can be quite different from those in adults. Delayed bone age and a delay in the onset of puberty are significant clinical markers. A child with hypothyroidism may also have a puffy face and a “younger” appearance than their actual age. If a goitre is present, it may be the first physical sign noticed by a parent or doctor. Regular monitoring and dose adjustments are required throughout childhood to ensure the levothyroxine dose keeps pace with the child’s rapid growth and changing metabolic needs. 

Secondary hypothyroidism involves a failure of the pituitary gland. 

While most causes of hypothyroidism are “primary” (the problem is in the thyroid itself), a small number of adults and children have “secondary” hypothyroidism. This occurs when the pituitary gland in the brain fails to produce enough Thyroid Stimulating Hormone (TSH). Without the TSH signal, a healthy thyroid gland will not produce its own hormones. This can be caused by a pituitary tumour, a head injury, or a genetic condition that affects the development of the brain’s endocrine centres. 

In children, secondary hypothyroidism may be part of a broader condition where the pituitary gland fails to produce other essential hormones, such as growth hormone or cortisol. This requires a complex management plan involving a paediatric endocrinologist. In adults, it is often detected after a significant event like a stroke or a brain injury. Diagnosing secondary hypothyroidism is more complex because the TSH levels in the blood may appear “normal” or low, even though the body is clearly deficient in thyroid hormones, requiring the measurement of “Free T4” to confirm the diagnosis. 

Cause Type	Primary Examples (Adults)	Primary Examples (Children)
Autoimmune	Hashimoto’s Thyroiditis	Hashimoto’s (Acquired)
Developmental	N/A	Congenital Hypothyroidism
Medical/Iatrogenic	Surgery, Radioactive Iodine	Previous Cancer Treatment
Nutritional	Iodine Deficiency/Excess	Maternal Iodine Deficiency
Central/Secondary	Pituitary Tumours	Congenital Pituitary Issues

Conclusion 

The causes of hypothyroidism are diverse and depend heavily on the age of the individual. In adults, autoimmune disease and the consequences of medical treatments are the most frequent triggers, while children are more often affected by congenital development issues or autoimmune reactions during adolescence. Regardless of the cause, the resulting lack of thyroid hormones affects every organ system, from the heart to the brain. Early diagnosis through blood tests or newborn screening is the key to preventing long term health complications. If you or your child experience severe, sudden, or worsening symptoms, such as extreme lethargy, severe facial swelling, or a very slow heart rate, call 999 immediately. 

Authority Snapshot 

The purpose of this article is to provide the general public with an accurate, evidence based understanding of the diverse causes of hypothyroidism in both adults and children. The content has been authored and reviewed by Dr. Rebecca Fernandez, a UK trained physician with clinical experience in internal medicine and emergency care. All information and guidance provided are strictly aligned with the clinical standards established by the NHS, NICE, and the British Society for Paediatric Endocrinology and Diabetes.