Thyroid Hemiagenesis: Symptoms, Diagnosis, and Treatment

Thyroid hemiagenesis is a rare congenital condition where a person is born with only one of the two lobes of the thyroid gland. From birth, either the right or left side of the gland is absent, while the remaining lobe is present and functional. The single lobe can often produce sufficient hormones to regulate the body’s metabolism. Over time, this solitary lobe may enlarge to compensate for the missing half or develop its own distinct thyroid-related health conditions.

Understanding the Causes of Thyroid Hemiagenesis

Thyroid hemiagenesis originates during early fetal development, a process known as embryogenesis. This developmental anomaly occurs when the thyroid gland, which begins at the base of the tongue and descends to its final position in the neck, fails to form its two lobes correctly. The precise triggers for this are not fully understood, and most cases are sporadic, occurring randomly without a clear cause.

While most cases have no known origin, some evidence points toward a genetic component, especially when the condition appears in multiple family members. Mutations in genes that guide thyroid development, such as PAX8 and FOXE1, have been identified in a small number of patients, though a specific genetic link is rarely established. The absence of the left thyroid lobe is significantly more common, occurring about four times as often as the absence of the right. In approximately half of the cases where the left lobe is missing, the isthmus, the tissue connecting the two lobes, is also absent.

Recognizing Symptoms and Related Thyroid Issues

A majority of individuals with thyroid hemiagenesis are asymptomatic and may live their entire lives without knowing they have the condition. This is because the single lobe is often capable of producing adequate amounts of thyroid hormone, maintaining a normal functional state (euthyroidism). When symptoms do arise, it is because the solitary lobe is unable to meet the body’s metabolic demands, leading to hypothyroidism.

Should hypothyroidism develop, individuals may experience symptoms such as:

• Persistent fatigue
• Unexplained weight gain
• Increased sensitivity to cold
• Constipation
• Dry skin

The presence of only one lobe does not prevent it from developing other common thyroid problems. The solitary lobe may be more prone to certain conditions like a goiter (enlargement), nodules, or autoimmune disorders such as Hashimoto’s thyroiditis and Graves’ disease.

How Thyroid Hemiagenesis is Diagnosed

Thyroid hemiagenesis is most often discovered by chance during medical evaluations for unrelated issues. An individual might undergo neck imaging, such as a carotid ultrasound, CT scan, or MRI for a different reason, which then reveals the absence of one thyroid lobe. This is called an incidental finding, as the condition itself is not causing symptoms that would prompt an investigation.

Once suspected, a thyroid ultrasound is the primary method used to confirm the diagnosis. This non-invasive imaging technique provides a clear picture of the neck, showing the presence of one lobe and the empty space where the other should be. It also allows for a detailed assessment of the existing lobe’s size and structure. To verify that the absent lobe is not simply ectopic (in an abnormal location), a thyroid scintigraphy may be performed. This scan uses a radioactive tracer to visualize active thyroid tissue, confirming one lobe is missing.

Alongside imaging, blood tests are performed to assess the function of the remaining lobe. These tests measure levels of thyroid-stimulating hormone (TSH), as well as the thyroid hormones free T4 and free T3. If an autoimmune condition like Hashimoto’s or Graves’ disease is suspected, tests for specific thyroid antibodies will also be conducted.

Managing Thyroid Hemiagenesis and Long-Term Outlook

For individuals who are asymptomatic with normal thyroid hormone levels, no direct treatment is required. Management focuses on periodic monitoring to ensure the remaining lobe continues to function properly. This involves regular blood tests to check TSH levels and periodic thyroid ultrasounds to monitor the solitary lobe for the development of nodules or enlargement.

If hypothyroidism develops, treatment involves hormone replacement therapy. The standard treatment is a daily dose of levothyroxine, a synthetic thyroid hormone that restores normal hormone levels and resolves symptoms. Should other conditions arise in the remaining lobe, such as a goiter or nodules, those are managed according to standard clinical guidelines.

The long-term outlook for individuals with thyroid hemiagenesis is excellent. The condition itself does not affect life expectancy and does not diminish quality of life, particularly when thyroid function remains normal or is effectively managed with medication. With appropriate monitoring, people with a single thyroid lobe can expect to lead a healthy life.