How It Is Diagnosed: Scans and Tests

Diagnosing thyroid hemiagenesis often begins as a surprise. Because the condition rarely causes symptoms on its own, it is most frequently discovered “incidentally”—meaning a doctor finds it while looking for something else ^[1][2]. To confirm that a lobe is truly missing from birth, doctors use a combination of imaging and blood tests to rule out other possibilities.

Imaging: Seeing the Structure

The first step in diagnosis is almost always an ultrasound. This test uses sound waves to create a picture of the thyroid gland.

• Initial Discovery: On an ultrasound, a radiologist will look for the two lobes of the thyroid. In hemiagenesis, one lobe is simply not there ^[3].
• The “Isthmus” Clue: The isthmus is the thin bridge of tissue that normally connects the two lobes. In many cases of congenital hemiagenesis, the isthmus is missing along with the lobe ^[4]. However, surgeons also routinely remove the isthmus during a hemithyroidectomy (surgical removal of a lobe). Therefore, an absent isthmus alone doesn’t prove you were born this way.
• Ruling Out Prior Surgery: The most definitive way doctors distinguish congenital hemiagenesis from a prior surgery is through your medical history and a physical exam looking for surgical scars. Additionally, an ultrasound of a previously operated neck often shows “surgical beds” or scar tissue that aren’t present in a person born with only one lobe ^[3].

Scintigraphy: Checking for Function

While an ultrasound shows the structure of the gland, a thyroid scintigraphy (or thyroid scan) shows its function. During this test, a tiny amount of a radioactive tracer (usually Technetium-99m or Iodine-123) is injected or swallowed. A special camera then maps where the thyroid tissue is active ^[5][6].

Note: It is completely normal to feel apprehensive about receiving a “radioactive tracer.” Please be reassured that the dose of radiation used in these scans is incredibly small, strictly regulated, and very safe for routine testing.

This scan is often considered a reliable tool for two reasons:

1. Confirming Absence: It proves that there is no functional thyroid tissue on the missing side ^[7].
2. Finding Ectopic Tissue: Sometimes, a thyroid lobe isn’t missing; it just didn’t move to the right place during fetal development. This is called ectopic thyroid tissue. A scan can find this tissue if it is “hiding” at the back of the tongue (lingual thyroid) or lower in the chest ^[8][9].

Blood Tests: Checking the “Workload”

Once the diagnosis is confirmed, the next step is to see how well the remaining lobe is performing. Doctors use Thyroid Function Tests (TFTs):

• TSH (Thyroid Stimulating Hormone): This hormone is produced by the pituitary gland (a small gland at the base of the brain). It acts as the “messenger” telling your thyroid to make hormones. To keep your hormone levels stable, your pituitary gland may release slightly more TSH to signal your single lobe to step up production ^[8]. Importantly, this slightly higher TSH is often still completely within the normal laboratory range ^[10].
• Free T4 and T3: These are the actual hormones produced by the thyroid.

Most people with one lobe are euthyroid, meaning their Free T4 and T3 hormone levels are perfectly normal, and they have no symptoms ^[10].

However, because you only have “half the factory,” your doctor will likely monitor these levels over time for two reasons: to ensure the remaining lobe isn’t struggling to meet the body’s needs (which would lead to hypothyroidism), and to help detect independent autoimmune conditions, like Hashimoto’s thyroiditis, which you may be at slightly higher risk for ^[11][2].