What Is Thyroid Testing?
These are the lab tests we can run to see how well your thyroid is working. In this post we will look at some of the common lab tests to assess the function of thyroid gland.
Thyroid function tests
Thyroid function tests are commonly ordered tests for thyroid testing. The most commonly ordered thyroid function tests performed on blood samples are as follows:
- Serum TSH: This is the best initial test for determining thyroid function.
- Serum T4 tests: T4 tests are considered more useful than T3 tests because T4 levels change more rapidly than T3 levels and thus are more sensitive to alterations in the disease course.
- Total T4: This test measures the total amount of T4 present within the bloodstream and includes both bound (to plasma protein) and unbound (free) forms.
- Free T4: Free T4 represents the amount of T4 present in the blood that is not bound to the plasma proteins. Free T4 is biologically active. This test in combination with serum TSH is most useful in diagnosing abnormal thyroid function.
- Serum T3: T3 is the metabolic product of T4. T3 is the bioactive form. Measuring T3 levels is most useful when evaluating a patient with hyperthyroidism and can help indicate the severity of hyperthyroidism.
- Total T3: This test determines the total amount of T3 present within the bloodstream and includes both bound and unbound forms of T3.
- Free T3: Free T3 represents the amount of T3 present in the blood that is not bound to any plasma proteins and thus biologically active.
Here is a table that gives you a fair idea of test results you can expect for common thyroid abnormalities:
| Condition | TSH Levels | T4 Levels | T3 Levels |
|---|---|---|---|
| Primary hypothyroidism | Elevated | Decreased | Decreased |
| Primary hyperthyroidism | Decreased | Elevated | Elevated (degree of elevation may indicate severity) |
| Secondary hypothyroidism | Decreased | Decreased | Decreased |
| Secondary hyperthyroidism | Elevated | Elevated | Elevated |
| Subclinical hypothyroidism | Elevated | Normal | Normal |
| Subclinical hyperthyroidism | Decreased | Normal | Normal |
| Exogenous hyperthyroidism | Decreased | Elevated | Elevated |
Thyroid Antibody Tests
Often patients might be having conditions in which self-reactive antibodies damage thyroid tissue. These antibody tests can help us in differentiating specific causes of hypothyroidism and hyperthyroidism. They are an important part of thyroid testing.
There are two antibodies that could be potentially problematic:
- Antithyroid peroxidase antibodies: They are found in patients with Hashimoto thyroiditis. These antibodies cause inflammatory destruction of the thyroid tissue. Initially, patients may present with a hyperthyroid state because of the release of preformed thyroid hormone that is stored in follicles into the circulation. This is later followed by a state of persistent hypothyroidism.
- Anti-TSH receptor antibodies: They are found in patients with Graves disease. Graves disease is an example of a type 2 hypersensitivity reaction. The antibody binds to the TSH receptor and stimulates the production of thyroid hormone.
Radioactive Iodine Uptake Scan
This test (radioactive iodine uptake scan: thyroid testing) is generally used to determine a specific cause for hyperthyroidism. This test is not used on pregnant patients or breastfeeding mothers.
How does it work?
Isotope used: radioactive iodine I-123
The patient is given an iodine radioisotope (123I) pill 24 hours prior to the scan. Then the thyroid’s uptake of the radiotracer is evaluated by an imaging device.
Interpreting radioactive iodine uptake scan
| Result | Interpretation |
|---|---|
| Low or absent uptake | 1) can suggest primary inflammation or destruction of the thyroid tissue (thyroiditis) 2) may also suggest factitious (artificial) thyrotoxicosis (hyperthyroidism), which most commonly occurs because of excessive or inappropriate use of thyroid hormone replacement medication. 3) may also rarely indicate that there is another place in the body producing thyroid hormone. Eg: struma ovarii |
| High to normal uptake | there is a new production of thyroid hormone occurring within the thyroid, as seen in Graves disease or toxic multinodular goiter. |
Location of Iodine uptake:
If uptake is increased as seen in the radioiodine uptake scan(thyroid testing), we can further use the test to identify whether hyperthyroidism is due to nodular or diffuse cause. A thyroid adenoma will be seen as a focal point (single spot) whereas Graves disease is seen as diffuse uptake of radioactive iodine.
Let us look at pictures of radio uptake scans of patients suffering with various conditions:
Tumor Marker tests
Tumor markers are compounds that are elevated in the blood of the patient when he/she is having specific cancers. Different thyroid cancers have different markers based on their metabolism, what they produce, the cell type from which they originated, and their locations.
The following table gives you a brief idea of various tumor markers that can be used to monitor the progression and recurrence of various types of thyroid cancer.
| Tumor Marker | Normal Physiological Function of the Marker | Associated Thyroid Cancer and Use |
|---|---|---|
| Calcitonin | Hormone normally produced by the thyroid parafollicular C cells. Active in bone and calcium homeostasis. | Medullary thyroid cancer recurrence. |
| Thyroglobulin | Protein used in the transport of tyrosine residues in the production of thyroid hormone. | Papillary or follicular thyroid cancer recurrence. |
| Carcinoembryonic antigen (CEA) | Family of glycoproteins involved in cell adhesion. Normally produced in utero by gastrointestinal tissue. | Medullary thyroid cancer recurrence. Nonspecific: also seen in cancers of the colon, pancreas, lung, and breast. |
Note
Anaplastic carcinoma of thyroid is not associated with any specific marker.
Thyroid Biopsy
- Can help us determine if a nodule is benign or malignant.
- Can also help us in diagnosing the type of thyroid cancer.
- Biopsy is done rarely for non-nodular thyroid enlargement.
- Thyroid biopsy is often done along with thyroid ultrasonography
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