The thyroid gland is a small, butterfly-shaped gland located at the base of the neck. It produces thyroid hormones that regulate metabolism and affect nearly every organ system in the body. Thyroid disorders are common, affecting around 12% of the population. The most common thyroid disorders include hypothyroidism, hyperthyroidism, Hashimoto’s thyroiditis, and thyroid nodules or goiters.
Thyroid function tests are important diagnostic tools that help determine if the thyroid is functioning properly. They measure the levels of thyroid hormones and antibodies in the blood. The main thyroid hormones are T3 (triiodothyronine) and T4 (thyroxine). The most important thyroid tests include:
TSH test
TSH stands for thyroid stimulating hormone. This test measures the amount of TSH in the blood. TSH is made by the pituitary gland in the brain. When thyroid hormone levels are low, the pituitary releases more TSH to stimulate the thyroid to make more hormones.
High TSH indicates hypothyroidism or underactive thyroid. Low TSH indicates hyperthyroidism or overactive thyroid. Normal TSH levels rule out primary thyroid disorders. The TSH test is considered the best and most sensitive indicator of thyroid health. It is usually the first test ordered when screening for thyroid problems.
Free T4 test
This test measures the level of free or unbound T4 hormone circulating in the bloodstream. Free T4 levels decrease when the thyroid is underactive (hypothyroidism) and increase when it is overactive (hyperthyroidism). The free T4 test is often ordered along with TSH to confirm and determine the severity of hyper or hypothyroidism.
Total T3 test
This test measures both bound and free T3 levels. T3 is the more active thyroid hormone. Total T3 testing may be helpful when TSH and free T4 levels seem inconsistent with the patient’s symptoms.
Thyroid antibodies tests
These blood tests check for antibodies that attack the thyroid. High levels indicate an autoimmune thyroid disorder like Hashimoto’s or Graves’ disease. Two main types of thyroid antibodies tests are:
– Thyroid peroxidase antibodies (TPOAb)
– Thyroglobulin antibodies (TgAb)
Monitoring thyroid antibody levels over time can help diagnose the cause of thyroid problems.
When should thyroid tests be done?
Thyroid tests may be done for the following reasons:
Screening high risk groups
Certain people are more likely to develop thyroid problems, including:
– Women, especially over age 50
– People with a family history of thyroid disease
– Patients with autoimmune disorders
– Those with a goiter or thyroid nodules
– People with unexplained weight changes, fatigue, or other suspicious symptoms
Monitoring thyroid disease
People diagnosed with hypothyroidism, hyperthyroidism, or thyroid cancer need regular testing to monitor thyroid status and adjust treatment as needed.
Monitoring thyroid drug therapy
Patients taking synthetic thyroid hormones for hypothyroidism need periodic blood tests to ensure they are getting the proper dose. Too much medication can cause hyperthyroidism. Too little can lead to continued hypothyroidism.
Evaluating hyperthyroidism
TSH, T4, T3, and antibody testing helps determine the cause of an overactive thyroid. Causes include Graves’ disease, toxic adenomas, and thyroiditis.
Evaluating hypothyroidism
Testing helps diagnose the type of hypothyroidism. Hashimoto’s disease and iodine deficiency are common causes. It also allows doctors to determine the proper thyroid hormone replacement dose.
Investigating thyroid nodules
Thyroid testing helps determine if nodules are producing too much thyroid hormone and guides biopsy recommendations.
Monitoring thyroid cancer
Thyroid function tests are used to evaluate the success of cancer treatment and check for recurrence.
TSH reference ranges
TSH reference ranges may vary slightly between labs but are generally:
0.5 to 4.5 or 5 μU/mL for adults
1.7 to 9 μU/mL for newborns
0.7 to 5.7 μU/mL for children
TSH levels below 0.5 μU/mL usually indicate hyperthyroidism. Levels above 4.5 or 5 μU/mL generally indicate hypothyroidism. However, normal TSH levels differ slightly in certain groups:
| Group | TSH reference range |
|---|---|
| Pregnant women | 0.1 – 2.5 μU/mL |
| Teens going through puberty | 0.5 – 4.3 μU/mL |
| Women using oral contraceptives | 0.2 – 4.8 μU/mL |
| Postmenopausal women | 0.4 – 7.0 μU/mL |
Doctors may use age and condition-specific reference ranges when interpreting results. Mild TSH elevations outside the normal limits are not always indicative of hypothyroidism in certain groups.
Normal and abnormal thyroid levels
| Test | Normal range | Hypothyroidism | Hyperthyroidism |
|---|---|---|---|
| TSH | 0.5-4.5 μU/mL | High | Low |
| Free T4 | 0.7-1.8 ng/dL | Low | High |
| Total T3 | 100-200 ng/dL | Low | High |
| TPO Antibodies | High | Normal/High |
Results outside the normal range confirm a suspected thyroid disorder. Borderline results may require confirmation through additional testing.
Most sensitive thyroid test
The TSH test is considered the most sensitive marker of thyroid status. TSH levels change before free T4 and T3 levels when the thyroid is not functioning properly.
Even a slight TSH increase above the upper limit of normal indicates possible hypothyroidism. A slight decrease below the lower limit points to possible hyperthyroidism. For this reason, the TSH test is the preferred initial test to screen for thyroid disorders.
Advantages of the TSH test:
- Highly sensitive to subtle thyroid dysfunction
- Elevated TSH detects hypothyroidism up to 5 years before symptoms appear
- Cost-effective means of screening
- Requires a small blood sample
- Easy to obtain through routine blood work
If the TSH is abnormal, free T4 and potentially T3 levels are tested to confirm the diagnosis and determine the thyroid dysfunction severity.
Other common thyroid tests
Free thyroxine (Free T4)
This measures the unbound T4 hormone available for use by cells. Free T4 helps confirm hyper or hypothyroidism indicated by abnormal TSH. It may be the only abnormal level in mild thyroid disease when TSH and T3 are still within their reference range.
Total triiodothyronine (Total T3)
This test measures both free and protein-bound T3 in circulation. T3 is the active thyroid hormone that affects metabolic processes. Total T3 provides additional information in diagnosing hyperthyroidism or low thyroid states.
Thyroid peroxidase antibodies (TPOAb)
TPOAb tests identify autoimmune thyroiditis, the most common cause of hypothyroidism. Elevated levels confirm Hashimoto’s disease. TPOAb may also be ordered during pregnancy alongside TSH to screen for undiagnosed thyroid problems.
Thyroglobulin antibodies (TgAb)
TgAb is used to evaluate autoimmune thyroiditis and differentiate types of thyroiditis. It is also used along with TPOAb to monitor treatment response in Hashimoto’s disease. TgAb can remain elevated even after the thyroid gland is destroyed or removed.
Thyroid ultrasound
Thyroid ultrasounds create images of the thyroid to identify nodules, inflammation, and anatomical abnormalities. Ultrasound is often done along with thyroid function tests, especially when examining thyroid nodules that may require biopsy.
Radioactive iodine uptake (RAIU)
This nuclear imaging test looks at how much iodine the thyroid absorbs from the bloodstream. It helps evaluate the cause of hyperthyroidism before starting treatment. RAIU is also used to examine thyroid nodules and monitor for thyroid cancer recurrence after treatment.
Conclusion
Thyroid function tests play a critical role in detecting and managing thyroid disorders. The TSH blood test stands out as the most sensitive indicator of thyroid status and the preferred initial screening test. Free T4, total T3, thyroid antibodies, imaging, and RAIU provide additional valuable information when investigating the cause of thyroid problems. Ongoing testing ensures proper treatment and follow-up. Being familiar with the major thyroid tests allows for better understanding of results and overall thyroid health.