TSH

What is being tested?
This test measures the concentration of thyroid-stimulating hormone (TSH) in the blood. TSH is one of the hormones produced by the pituitary gland. Through a feedback regulation system, it helps keep the blood levels of the thyroid hormones thyroxine (T4) and triiodothyronine (T3) stable. When thyroid hormone levels in the blood fall, the hypothalamus releases thyrotropin-releasing hormone (TRH), which triggers the release of TSH from the pituitary, which in turn stimulates the thyroid to produce and release T3 and T4. When all three glands are working properly, thyroid activity keeps thyroid hormone levels constant.
Abnormal pituitary function causes changes in TSH levels. If they rise, the thyroid releases too much T4 and T3, and a clinical picture of hyperthyroidism may be seen. If TSH levels fall, thyroid hormone production falls and symptoms of hypothyroidism may appear.
A clinical picture of hypothyroidism or hyperthyroidism can also occur with abnormal hypothalamic function (excess or lack of TRH), and also in association with various thyroid diseases that alter thyroid hormone production regardless of blood TSH levels.

How is the sample collected for testing?
The blood sample is obtained from a vein or, in newborns, by heel prick.

How is it used?
When is it requested?
What does the test result mean?
Is there anything else I should know?

How is it used?
TSH is often the test of choice for assessing thyroid function and/or symptoms of hyperthyroidism or hypothyroidism. It is often requested together with, or before, T4. Other tests, such as T3 and thyroid antibodies, may be prescribed together with TSH or after an abnormal TSH result.

The TSH test is used to:

– diagnose a thyroid disorder when symptoms are present,
– screen newborns for hypothyroidism,
– monitor hormone therapy in people with hypothyroidism,
– diagnose and monitor female infertility problems,
– assess pituitary gland function (occasionally),
– screen thyroid function in the adult population.

When is it requested?
TSH is prescribed when there are symptoms of hyperthyroidism or hypothyroidism or when the thyroid gland is enlarged.
It is prescribed at regular intervals to monitor the effectiveness of treatment for abnormal thyroid function. While newborn screening is carried out for all babies, there is no consensus on using TSH as a screening test for thyroid function in adults.
In the case of abnormal thyroid function in pregnancy, it is prescribed in the early and late stages of pregnancy and also after delivery to monitor the mother and the baby.

What does the test result mean?
A high TSH value indicates an underactive thyroid gland that is not responding adequately to TSH stimulation because of chronic or acute thyroid disorders. Rarely, it is associated with a pituitary problem (secondary hyperthyroidism), such as a TSH-producing tumour. High TSH levels may be found in patients with a known thyroid problem (or who have had a thyroidectomy) who are receiving insufficient hormone therapy.
Low TSH levels may indicate an overactive gland (hyperthyroidism) or excessive hormone therapy in patients with an underactive thyroid or who have had a thyroidectomy. Rarely, a low TSH level indicates damage to the pituitary gland.
Abnormal TSH levels indicate an abnormal availability of thyroid hormones but do not identify the cause. An abnormal TSH result is therefore usually followed by further tests.

The following table summarises the test results and what they may mean:

TSH	T4	T3	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	Rare pituitary (secondary) hypothyroidism

Is there anything else I should know?
Many types of medicine – including aspirin and thyroid hormone replacement therapy – can affect thyroid function test results. It is therefore advisable to tell your doctor about any medicines you are taking before the test.
Once the dose of thyroid hormone therapy has been set, it is important to wait one or two months before repeating the TSH test to assess the effectiveness of the new dose.
Severe stress and acute illness can affect TSH test results.
Levels may be low in the first trimester of pregnancy.

1. Is the TSH test prescribed in pregnancy?

1. Is the TSH test prescribed in pregnancy?
It is not usually prescribed if there are no symptoms, but it is prescribed periodically when there are symptoms and/or known thyroid disease, to detect and assess hyperthyroidism or hypothyroidism and any effects on the mother and baby.