Hypothyroidism, also known as underactive thyroid disease, is a condition where the thyroid gland fails to produce enough thyroid hormones to meet the body’s needs. These hormones primarily thyroxine (T4) and triiodothyronine (T3) play a crucial role in regulating metabolism, body temperature, heart rate, and many other physiological functions. When thyroid hormone levels are too low, it can result in fatigue, weight gain, depression, dry skin, constipation, cold intolerance, and menstrual irregularities.

When thyroid hormone levels drop, the body’s processes begin to slow down. This can lead to a wide range of symptoms, such as persistent fatigue, unexplained weight gain, depression, Xeroderma, constipation, sensitivity to cold, and menstrual irregularities in women. Because many of these symptoms can be mistaken for signs of aging or stress, hypothyroidism often goes undiagnosed or misdiagnosed for years.

Causes of Hypothyroidism

Autoimmune Thyroiditis (Hashimoto’s Thyroiditis)

The leading cause of hypothyroidism worldwide, especially in developed countries, is Hashimoto’s thyroiditis an autoimmune condition. In this disorder, the body’s immune system mistakenly attacks the thyroid gland, leading to inflammation and gradual destruction of thyroid tissue.

Over time, the damage impairs the gland’s ability to produce sufficient thyroid hormones. Hashimoto’s thyroiditis tends to run in families and is more common in women, particularly those between 30 and 50 years old. It often develops slowly and may go unnoticed for years until symptoms become severe enough to seek medical attention.

The exact reason why the immune system turns against the thyroid in Hashimoto’s remains unclear, but genetic predisposition, environmental triggers (like viruses or stress), and hormonal factors are believed to contribute.

Iodine Deficiency

Iodine is a trace element essential for the production of thyroid hormones. When the body doesn’t get enough iodine from the diet, the thyroid cannot synthesize adequate levels of T3 and T4, leading to hypothyroidism.

Although iodine deficiency has become less common in many parts of the world due to iodized salt and public health initiatives, it remains a significant cause of hypothyroidism in certain regions of Africa, Asia, and Latin America. Pregnant women and people who avoid iodized salt are particularly at risk.

Interestingly, both too little and too much iodine intake can disrupt thyroid function. Excessive iodine intake in susceptible individuals may even trigger autoimmune thyroid disease.

Thyroid Surgery or Radioactive Iodine Therapy

Individuals who have undergone surgery to remove all or part of the thyroid gland a procedure known as thyroidectomy are at risk for developing hypothyroidism. This surgery is often performed to treat thyroid nodules, thyroid cancer, or hyperthyroidism.

Similarly, treatment for overactive thyroid (hyperthyroidism) using radioactive iodine therapy (RAI) can also lead to hypothyroidism. RAI works by damaging or destroying the overactive thyroid cells, which can inadvertently reduce hormone production too much, tipping the scale toward an underactive state.

In both cases, patients usually require lifelong thyroid hormone replacement therapy to maintain normal metabolic function.

Congenital Hypothyroidism

In some cases, babies are born with an underactive or absent thyroid gland, a condition known as congenital hypothyroidism. If left untreated, it can lead to severe developmental delays, intellectual disability, and stunted growth.

Congenital hypothyroidism may result from a variety of causes, including genetic mutations, problems in thyroid development during gestation, or a family history of thyroid dysfunction. Routine newborn screening programs in many countries now help detect this condition early so that prompt treatment can begin.

Fortunately, with early diagnosis and treatment, children with congenital hypothyroidism can develop normally and lead healthy lives.

Medications That Affect Thyroid Function

Certain medications can interfere with the normal functioning of the thyroid gland or reduce hormone production. One common example is lithium, a drug used to treat bipolar disorder. Lithium has been shown to inhibit thyroid hormone release and may cause hypothyroidism in susceptible individuals.

Other medications known to affect the thyroid include :-

Amiodarone (used for heart rhythm disorders), which contains high levels of iodine.
Interferon-alpha and interleukin-2, used in cancer and hepatitis treatment.
Tyrosine kinase inhibitors, such as sunitinib or sorafenib, used in cancer therapy.

Patients taking these medications should have their thyroid function monitored regularly to detect any early signs of dysfunction.

Pituitary or Hypothalamic Disorders

Though rare, hypothyroidism can also be caused by problems in the pituitary gland or the hypothalamus, which are responsible for regulating thyroid function through hormonal signals.

The pituitary gland produces thyroid-stimulating hormone (TSH), which tells the thyroid to make more T3 and T4. If the pituitary fails to produce adequate TSH due to a tumor, trauma, or certain diseases, thyroid hormone production may decrease. This form of hypothyroidism is called secondary hypothyroidism.

If the issue lies in the hypothalamus, which produces thyrotropin-releasing hormone (TRH) to stimulate the pituitary, it’s called tertiary hypothyroidism. These forms are uncommon but highlight the complex endocrine network involved in thyroid regulation.

Postpartum Thyroiditis

Some women experience thyroid inflammation after giving birth, known as postpartum thyroiditis. This condition can initially cause hyperthyroidism, followed by hypothyroidism within a few months.

It’s believed to be an autoimmune reaction triggered by the hormonal changes and immune system shifts that occur during and after pregnancy. While the thyroid function often returns to normal within 12 to 18 months, some women may develop permanent hypothyroidism.

Women with type 1 diabetes or a family history of thyroid issues are at higher risk of developing postpartum thyroiditis.

Radiation Exposure

Exposure to radiation, particularly to the head and neck area, can impair thyroid function. This includes :-

Radiation therapy for cancers like Hodgkin’s lymphoma or head and neck tumors.
Environmental or accidental exposure to radioactive materials.

Radiation damages the thyroid cells, reducing their ability to produce hormones over time. Individuals who have undergone such treatments should be monitored regularly for thyroid function abnormalities.

Chronic Illness and Aging

Chronic medical conditions, including chronic kidney disease, liver failure, and autoimmune disorders like lupus, may indirectly affect thyroid hormone metabolism and lead to hypothyroid-like symptoms or true hormone deficiency.

Additionally, aging is a contributing factor. As people get older, thyroid hormone levels may gradually decline. Although not all elderly individuals develop clinical hypothyroidism, subclinical forms (where TSH is elevated but T3 and T4 remain normal) become more common with age.

Dietary and Lifestyle Factors

Certain foods and nutritional imbalances can influence thyroid function. While not a primary cause of hypothyroidism in most cases, goitrogens substances found in foods like soy, cabbage, broccoli, and millet can interfere with iodine uptake when consumed in large quantities, especially in iodine-deficient individuals.

Additionally, selenium deficiency, excessive soy intake, or an extremely restrictive diet lacking essential nutrients can stress the thyroid gland and contribute to dysfunction.

Conclusion

Hypothyroidism is a complex condition with various underlying causes from autoimmune disease and iodine deficiency to surgery, medications, and even congenital factors. Identifying the root cause is critical for tailoring the right treatment approach and preventing complications.