Thyroid stimulation hormone (TSH) test: indication and interpretation

When to order this TSH test ?

The thyroid-stimulating hormone (TSH) test is often the test of choice for evaluating thyroid function and/or symptoms of a thyroid disorder, including hyperthyroidism or hypothyroidism.

TSH is produced by the pituitary gland, a tiny organ located below the brain and behind the sinus cavities. It is part of the body's feedback system to maintain stable amounts of the thyroid hormones thyroxine (T4) and triiodothyronine (T3) in the blood and to help control the rate at which the body uses energy.

A TSH test is frequently ordered along with or preceding a free T4 test. Other thyroid tests that may be ordered include a free T3 test and thyroid antibodies (if autoimmune-related thyroid disease is suspected). Sometimes TSH, free T4 and free T3 are ordered together as a thyroid panel.

What does the test result mean?

A high TSH result may mean that:

• The person tested has an underactive thyroid gland that is not responding adequately to the stimulation of TSH due to some type of acute or chronic thyroid dysfunction
• A person with hypothyroidism or who has had their thyroid gland removed is receiving too little thyroid hormone replacement medication and the dose may need to be adjusted
• A person with hyperthyroidism is receiving too much anti-thyroid medication and the dose needs adjusting
• There is a problem with the pituitary gland, such as a tumor producing unregulated levels of TSH

A low TSH result may indicate:

• An overactive thyroid gland (hyperthyroidism)
• Excessive amounts of thyroid hormone medication in those who are being treated for an underactive (or removed) thyroid gland
• Insufficient anti-thyroid medication in a person being treated for hyperthyroidism; however, it may take a while for TSH production to resume after successful anti-thyroid treatment. This is why the American Thyroid Association recommends monitoring this treatment with tests for thyroid hormones (T4 and T3) as well as TSH levels.
• Damage to the pituitary gland that prevents it from producing adequate amounts of TSH

Whether high or low, an abnormal TSH indicates an excess or deficiency in the amount of thyroid hormone available to the body, but it does not indicate the reason why. An abnormal TSH test result is usually followed by additional testing to investigate the cause of the increase or decrease.

The following table summarizes some examples of typical test results and their potential meaning.

TSH	FREE T4	FREE OR TOTAL T3	PROBABLE INTERPRETATION
High	Normal	Normal	Mild (subclinical) hypothyroidism
High	Low	Low or normal	Hypothyroidism
Low	Normal	Normal	Mild (subclinical) hyperthyroidism
Low	High or normal	High or normal	Hyperthyroidism
Low	Low or normal	Low or normal	Non-thyroidal illness; pituitary (secondary) hypothyroidism
Normal	High	High	Thyroid hormone resistance syndrome (a mutation in the thyroid hormone receptor decreases thyroid hormone function)

See more at: https://labtestsonline.org/understanding/analytes/tsh/tab/test/

Thyroid gland
Picture credit: www.medicinenet.com

free T4 (FT4) test : indication and interpretation

When to order free T4 test ?

Free thyroxine (free T4) tests are used to help evaluate thyroid function and diagnose thyroid diseases, including hyperthyroidism and hypothyroidism, usually after discovering that the thyroid stimulating hormone (TSH) level is abnormal.

T4 and another hormone called triiodothyronine (T3) are produced by the thyroid gland. They help control the rate at which the body uses energy and are regulated by a feedback system. TSH stimulates the production and release of T4 (primarily) and T3 from the thyroid gland.

Most of the T4 and T3 circulates in the blood bound to protein, while a small percentage is free (not bound). Blood tests can measure total T4, free T4, total T3, or free T3. The total T4 test has been used for many years, but it can be affected by the amount of protein available in the blood to bind to the hormone. Free T4 is not affected by protein levels and is the active form of thyroxine. The free T4 test is thought by many to be a more accurate reflection of thyroid hormone function and, in most cases, its use has replaced that of the total T4 test.

What does the test result mean?

In general, high free T4 results may indicate an overactive thyroid gland (hyperthyroidism), and low free T4 results may indicate an underactive thyroid gland (hypothyroidism). The test results alone are not diagnostic but will prompt the health practitioner to perform additional testing to investigate the cause of the excess or deficiency.

Both decreased and increased free T4 results are associated with a variety of temporary and chronic thyroid conditions. Low free T4 results in conjunction with a low TSH level or high free T4 results along with a high TSH may indicate a pituitary gland condition.

Interpretation:

TSH	FREE T4	FREE OR TOTAL T3	PROBABLE INTERPRETATION
High	Normal	Normal	Mild (subclinical) hypothyroidism
High	Low	Low or normal	Hypothyroidism
Low	Normal	Normal	Mild (subclinical) hyperthyroidism
Low	High or normal	High or normal	Hyperthyroidism
Low	Low or normal	Low or normal	Non-thyroidal illness; rare pituitary (secondary) hypothyroidism
Normal	High	High	Thyroid hormone resistance syndrome (a mutation in the thyroid hormone receptor decreases thyroid hormone function)

See more at: https://labtestsonline.org/understanding/analytes/t4/tab/test/

  A woman does a thyroid neck check in the mirror.
          Picture credit: www.medicinenet.com

free T3 test (FT3): indication and interpretation

When to order free T3 test ?

A free T3 or total T3 test may be ordered when someone has an abnormal TSH test result. It may be ordered as part of the investigative workup when a person has symptoms suggesting hyperthyroidism, especially if the free T4 level is not elevated.

Signs and symptoms may include:

• Increased heart rate
• Anxiety
• Weight loss
• Difficulty sleeping
• Tremors in the hands
• Weakness
• Diarrhea (sometimes)
• Light sensitivity, visual disturbances
• The eyes may be affected: puffiness around the eyes, dryness, irritation, and, in some cases, bulging of the eyes.

Free or total T3 may sometimes be ordered at intervals to monitor a known thyroid condition and to help monitor the effectiveness of treatment for hyperthyroidism.

What does the test result mean?

Increased or decreased thyroid hormone results indicate that there is an imbalance between the body's requirements and supply, but they do not tell the health practitioner specifically what is causing the excess or deficiency.

The following table summarizes some examples of typical test results and their potential meaning.

TSH	FREE T4	FREE OR TOTAL T3	PROBABLE INTERPRETATION
High	Normal	Normal	Mild (subclinical) hypothyroidism
High	Low	Low or normal	Hypothyroidism
Low	Normal	Normal	Mild (subclinical) hyperthyroidism
Low	High or normal	High or normal	Hyperthyroidism
Low	Low or normal	Low or normal	Non-thyroidal illness; rare pituitary (secondary) hypothyroidism
Normal	High	High	Thyroid hormone resistance syndrome (a mutation in the thyroid hormone receptor decreases thyroid hormone function)

If someone is being treated with anti-thyroid medication for hyperthyroidism and the free or total T3 (or more frequently, the free T4 or TSH) is normal, then it is likely that the medication is effective in treating the condition. If the free or total T3 or free T4 is elevated, then the medication is not effective in treating the condition and the person may be experiencing symptoms associated with hyperthyroidism.

See more at: https://labtestsonline.org/understanding/analytes/t3/tab/test/

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