Causes of Hypothyroidism

The Most Common Causes of Hypothyroidism

Hypothyroidism, also known as an underactive thyroid or thyroid failure, can result from various common factors. Below are some of the primary causes:

Hashimoto’s Disease

Hashimoto’s disease, scientifically known as Hashimoto’s thyroiditis, is a prevalent autoimmune disorder characterized by the immune system attacking the thyroid gland. This leads to damage in the thyroid tissue and a reduced ability to produce hormones. Women are eight times more likely to develop Hashimoto’s than men. While the exact cause of Hashimoto’s remains undetermined, research suggests a correlation with a family history of the condition, and women experiencing postpartum thyroiditis may also be at increased risk. Additionally, individuals with other autoimmune disorders are at a higher risk of developing Hashimoto’s, including:

• Vitiligo.
• Addison’s disease.
• Type 1 diabetes.
• Sjögren’s syndrome.
• Autoimmune hepatitis.
• Pernicious anemia.
• Lupus.
• Celiac disease.
• Rheumatoid arthritis.

It is crucial to receive appropriate treatment for Hashimoto’s to manage the condition and prevent complications such as infertility, elevated cholesterol levels, congenital disabilities in untreated pregnant women, miscarriage, cardiovascular issues, and mental health disorders. In severe cases, individuals may experience myxedema, a rare but serious form of hypothyroidism that can lead to severe complications like seizures, heart failure, coma, and even death.

Medications

In some instances, hypothyroidism may result from certain medications. These drugs can cause temporary thyroid inactivity that resolves upon discontinuation or lead to chronic thyroid failure. Some of these medications include:

• Lithium: Often prescribed for various health conditions like certain blood disorders, schizophrenia, depression, and bipolar disorder, lithium can lead to hypothyroidism, particularly in individuals with existing thyroid issues or genetic predispositions.
• Iodine: Essential for hormone production, iodine deficiency can result in hypothyroidism. Conversely, excessive iodine from supplements may disrupt thyroid function, potentially causing hypothyroidism.
• Amiodarone: This medication, prescribed for cardiac arrhythmias, contains high levels of iodine and can lead to hypothyroidism due to thyroid inflammation, causing an initial hormone spike followed by a depletion.
• Interferon alpha: Used to treat certain cancers and hepatitis C, this drug may also cause thyroid inflammation and subsequent hypothyroidism.
• Other Medications: Other drugs like Bexarotene, Ipilimumab, and Interleukin-2 can also have similar effects.

Additionally, hypothyroidism can occur after aggressive treatment for hyperthyroidism, where radioactive iodine or antithyroid drugs significantly reduce hormone production, leading to permanent thyroid insufficiency.

Thyroidectomy

A patient may require surgery to partially or completely remove the thyroid gland, often performed to excise thyroid nodules, treat thyroid cancer, or alleviate symptoms from conditions like Graves’ disease. If part of the thyroid is removed, the risk of hypothyroidism ranges from 30-50%. Patients who have their entire thyroid removed will need lifelong thyroid hormone replacement therapy.

For more information on thyroidectomy, you may refer to the following article: (Thyroidectomy).

Radiation Therapy

Patients who undergo radiation therapy for head and neck cancers have an approximately 50% chance of developing hypothyroidism, as radiation can damage both cancerous and healthy thyroid cells.

Less Common Causes of Hypothyroidism

In addition to the previously mentioned causes, there are several less common factors that may contribute to hypothyroidism:

Congenital Hypothyroidism

Congenital hypothyroidism is a rare condition that arises due to developmental defects in the thyroid gland during embryonic development. In some instances, the gland may be present but incapable of producing hormones, a condition known as thyroid dyshormonogenesis. There is no identified cause for this genetic disorder, although screening at birth typically detects it before symptoms manifest.

Pituitary Disorders

Hypothyroidism can also arise from dysfunction in the pituitary gland, termed secondary hypothyroidism. Since the pituitary and hypothalamus regulate thyroid hormone production, failure to produce adequate thyroid-stimulating hormone (TSH) can lead to diminished thyroid activity. This condition may occur due to pituitary tumors or inflammation and tends to be more prevalent in individuals over 50 or those with a known history of pituitary disorders.

Pregnancy

Approximately 3% of pregnant women may experience hypothyroidism, usually as a result of Hashimoto’s disease. Thyroid hormones are vital for fetal growth during the first trimester, making treatment essential. Symptoms in pregnant women may include extreme fatigue, muscle cramps, constipation, cold intolerance, memory lapses, and concentration issues. Serious complications for both mother and baby in severe cases can include:

• Anemia.
• Miscarriage.
• Stillbirth.
• Low birth weight.
• Congestive heart failure.
• Preeclampsia.

For more details on hypothyroidism during pregnancy, refer to the following article: (Hypothyroidism in Pregnancy).

Some women may also develop postpartum thyroiditis, a rare condition marked by inflammation of the thyroid in the first year after giving birth. This typically starts with hyperthyroidism but often progresses to hypothyroidism as the thyroid’s hormone reserves wane. In most cases, the thyroid resumes normal function within a year, although some may experience chronic hypothyroidism, particularly those with type 1 diabetes.

For further information on postpartum thyroiditis, see this article: (Postpartum Thyroiditis).

Iodine Deficiency

Iodine is crucial for thyroid hormone production, and insufficient iodine intake can lead to disease symptoms analogous to hypothyroidism. Chronic iodine deficiency can contribute to various thyroid disorders that may result in hypothyroidism, including goiter and conditions like cretinism in fetuses exposed to low iodine levels, resulting in both mental and physical development issues. Several factors can lead to iodine deficiency:

• Inadequate iodine in the diet.
• Pregnancy.
• Living in regions with low iodine soil levels, such as mountainous areas.
• A diet low in iodine-rich foods.

Types of Hypothyroidism

Hypothyroidism can be classified into three main types:

• Primary Hypothyroidism: This type occurs when the thyroid gland loses its ability to produce sufficient hormones despite proper stimulation, indicating an inherent dysfunction in the gland itself.
• Secondary Hypothyroidism: In this case, the thyroid is inadequately stimulated, preventing adequate hormone production, so the problem lies not within the thyroid but in the pituitary gland or the hypothalamus due to pituitary disorders, tumors, or past surgeries.
• Tertiary Hypothyroidism: This condition primarily results from dysfunction within the hypothalamus.

Treatment of Hypothyroidism

The main approach to treating hypothyroidism involves restoring thyroid hormone levels to their normal range and managing associated symptoms. This is typically achieved through daily oral administration of levothyroxine, a synthetic thyroid hormone. Patients often experience improvement within a short period of starting the medication, although the duration of treatment varies. Some may require it temporarily, while others need lifelong therapy. Levothyroxine dosages are adjusted based on individual patient needs, and the following factors are crucial in the treatment process:

Determining the Appropriate Medication Dose

Doctors base the appropriate levothyroxine dosage on several factors, including age, cardiovascular status, and concurrent health conditions like pregnancy. Clinical response and laboratory results guide dose adjustments. Monitoring hormone levels, particularly free thyroxine, is essential, as TSH levels may not provide reliable readings. Generally, it takes about four to six weeks for levothyroxine to reach peak effectiveness. High doses can lead to side effects such as increased appetite, insomnia, palpitations, and tremors. For patients with coronary artery disease or severe hypothyroidism, lower starting doses may be better for gradual increase, allowing the body to adjust. Patients should not stop or adjust their dosage without consulting a doctor, as symptom recurrence can occur gradually.

The maximum recommended levothyroxine dosage for hypothyroidism is between 1-1.6 micrograms per kilogram per day, depending on absorption factors, while maximum daily doses typically do not exceed 200-300 micrograms. Needs for doses beyond these levels are rare and may indicate non-compliance, malabsorption, or drug interactions.

Levothyroxine Absorption

Certain medications, dietary supplements, and foods can impact levothyroxine absorption, making it essential to inform the doctor about any concurrent medications, excessive soy consumption, or high-fiber diets. Notable substances that may hinder absorption include:

• Iron supplements or multivitamins containing iron.
• Aluminum hydroxide, found in some antacids.
• Calcium supplements.

Levothyroxine should be taken on an empty stomach at the same time daily. If taken in the morning, patients should wait at least one hour before eating or taking other medications. If taken at night, they should wait a minimum of four hours after their last meal, regardless of the meal’s size. If a dose is missed, the patient should take their next dose as scheduled the following day.

Subclinical Hypothyroidism

Subclinical hypothyroidism is characterized by elevated thyroid-stimulating hormone (TSH) levels with normal free thyroxine levels. Managing this condition requires careful consideration, as merely increasing TSH slightly may not be beneficial and can sometimes be harmful. However, higher TSH levels may stimulate thyroid hormones, improving blood cholesterol levels and overall well-being. Treatment may be indicated for patients with TSH exceeding 10 micro-units per milliliter, especially if they show increasing thyroid antibodies or exhibit symptoms like fatigue, dry skin, constipation, or muscle cramps, with TSH levels ranging between 5-10 micro-units per milliliter.

For more information on hypothyroidism treatment, see the following article: (What is the Treatment for Hypothyroidism).