Laboratory medicine

Laboratory medicine

Monday, May 9, 2016

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.
HighNormalNormalMild (subclinical) hypothyroidism
HighLowLow or normalHypothyroidism
LowNormalNormalMild (subclinical) hyperthyroidism
LowHigh or normalHigh or normalHyperthyroidism
LowLow or normalLow or normalNon-thyroidal illness; pituitary (secondary) hypothyroidism
NormalHighHighThyroid hormone resistance syndrome (a mutation in the thyroid hormone receptor decreases thyroid hormone function)
Thyroid gland
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