Thyroid stimualting hormone (TSH) testing is used to:

• screen newborns for an underactive thyroid gland
• diagnose a thyroid disorder in a person with symptoms
• monitor thyroid replacement therapy in people with hypothyroidism
• as part of the investigation of female infertility problems
• help to evaluate pituitary function (occasionally)

Your doctor requests this test if you show symptoms of a thyroid disorder. For example, symptoms of hyperthyroidism include heat intolerance, weight loss, rapid heartbeat, nervousness, sweating and increased frequency of stools.

Common symptoms of hypothyroidism include fatigue, weakness, weight gain, muscle aches, constipation, slow heart rate and cold intolerance.

The blood test is often requested with other thyroid hormone tests and after a physical examination of your thyroid. TSH screening is routinely performed in newborns. There are currently no recommendations for routine screening of adults in Australia.

A high TSH result often means an underactive thyroid gland caused by failure of the gland (hypothyroidism). Very rarely, a high TSH result can indicate a problem with the pituitary gland, such as a tumour, in what is known as secondary hyperthyroidism. A high TSH value can also occur in people with underactive thyroid glands who have been receiving too little thyroid hormone medication.

A low TSH result can indicate an overactive thyroid gland (hyperthyroidism) or damage to the pituitary gland that prevents it from producing TSH. A low TSH result can also occur in people with an underactive thyroid gland who are receiving too much thyroid hormone medication.

An abnormal TSH usually indicates a deficiency or an excess of thyroid hormones available to the body but it does not indicate the reason why. An abnormal TSH result is usually followed by additional testing of free thyroxine (FT4) and/or free tri-iodothyronine (FT3) to investigate the cause. The combination of a TSH test and tests for the thyroid hormones T3 and T4 is known as a thyroid function test (TFT).

The following table summarises patterns of thyroid function test results and their most common causes.

TSH	FT4	FT3	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	Nonthyroidal illness; rare pituitary (secondary) hypothyroidism

About Reference Intervals

Thyroid-hormone replacement therapy and some drugs may interfere with thyroid function test results, so tell your doctor about any drugs you are taking.

When your doctor adjusts your dose of thyroid hormone, it is important that you wait at least one to two months before checking your TSH again, so that your new dose can have its full effect.

Extreme stress and acute illness may also affect TSH test results. In this event, TSH should be rechecked a few weeks after the illness has resolved. 

Results for TSH may be low during the first trimester of pregnancy. This is because one of the hormones of pregnancy (HCG) has a TSH-like effect so the pituitary is not required to produce as much TSH. TSH results in pregnancy should be interpreted in relation to reference intervals specific for pregnancy.

Hyperthyroidism can be controlled through treatment. This will normally involve tablets to stop the thyroid gland producing thyroid hormones, radioactive iodine treatment to destroy thyroid tissue, or surgery to remove part or all of the thyroid gland.

Hypothyroidism is easily treated and controlled for most people with thyroxine (T4) replacement in the form of a tablet.

Yes, that is a good way to test for hypothyroidism during pregnancy, which is often overlooked.

For more information, see British Thyroid Foundation: Thyroid in Pregnancy