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Diagnostic Tests 2023

TSH exam: what is it for and why is itá high or low

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TSH exam: what is it for and why is itá high or low
TSH exam: what is it for and why is itá high or low

The TSH test is used to evaluate thyroid function and is usually requested by the general practitioner or endocrinologist, to assess whether this gland is working properly, and in case of hypothyroidism, hyperthyroidism, or in the follow-up in case of differentiated cancer thyroid, such as follicular or papillary, for example.

Thyro-stimulating hormone (TSH) is produced by the pituitary gland and its purpose is to stimulate the thyroid to produce T3 and T4 hormones. When TSH values are increased in the blood, it means that the concentration of T3 and T4 in the blood is low. When found in low concentrations, T3 and T4 are present in high concentrations in the blood. See what are the essential tests to evaluate the thyroid.


Reference values

TSH reference values vary according to the age of the person and the laboratory in which the test is performed, being normally:

Age Values
1st week of life 15 (μIU/mL)
2nd week to 11 months 0, 8 – 6, 3 (μIU/mL)
1 to 6 years 0, 9 – 6.5 (μIU/mL)
7 to 17 years old 0, 3 – 4, 2 (μIU/mL)
Adults (+ 18 years) 0, 3 – 4.0 (μIU/mL)

In pregnancy, the values are different:

In pregnancy Values
1st quarter 0, 1 – 3.6 mIU/L (μIU/mL)
2nd quarter 0, 4 – 4.3 mIU/L (μIU/mL)
3rd quarter 0, 4 – 4.3 mIU/L (μIU/mL)

What the results can mean

High TSH

  • Hypothyroidism: Most of the time, high TSH indicates that the thyroid is not producing enough hormone, so the pituitary tries to compensate for this by increasing TSH levels in the blood for the thyroid to perform its function properly. One of the hallmarks of hypothyroidism is high TSH and low T4, and it can indicate subclinical hypothyroidism when TSH is high but T4 is within normal range. Find out what T4 is.
  • Medicines: The use of low doses of medications against hypothyroidism or other medications, such as Propranolol, Furosemide, Lithium and iodine medications, can increase the concentration of TSH in the blood.
  • Pituitary tumor can also cause increased TSH.

Symptoms related to high TSH are typical of hypothyroidism, such as tiredness, weight gain, constipation, feeling cold, increased facial hair, difficulty concentrating, dry skin, brittle hair and nails, and brittle. Learn more about hypothyroidism.

low TSH

  • Hyperthyroidism: Low TSH usually indicates that the thyroid is overproducing T3 and T4, increasing these values, and therefore the pituitary decreases the release of TSH to try regulate thyroid function. Understand what T3 is.
  • Use of medication: When the dose of medication against hypothyroidism is too high, TSH values are less than ideal. Other drugs that can cause low TSH are: AAS, corticosteroids, dopamine agonists, fenclofenac, heparin, metformin, nifedipine or pyridoxine, for example.
  • Pituitary tumor can also lead to low TSH.

Symptoms related to low TSH are typical of hyperthyroidism, such as restlessness, heart palpitations, insomnia, weight loss, nervousness, tremors, and decreased muscle mass. In this case, it is normal for TSH to be low, and T4 to be high, but if T4 is still between 01 and 04 μIU/mL, this may indicate subclinical hyperthyroidism. Low TSH and low T4, can indicate anorexia nervosa, for example, but in any case the diagnosis is given by the doctor who requested the exam. Learn more about treating hyperthyroidism.


How the TSH exam is done

The TSH test is performed from a small blood sample, which must be collected after fasting for at least 4 hours. The collected blood is sent to the laboratory for analysis.

The best time to do this test is in the morning, as the concentration of TSH in the blood varies throughout the day. Before performing the test, it is important to indicate the use of medication, especially thyroid medication, such as Levothyroxine, as it can interfere with the test result.

What is ultra sensitive TSH

The ultra sensitive TSH test is a more advanced diagnostic method that can detect minute amounts of TSH in the blood, which a normal test would not be able to identify. The diagnostic method used in laboratories is very sensitive and specific, and the ultra-sensitive TSH test is normally used in routine.

When the TSH exam is requested

The TSH test may be ordered in he althy people, only to assess thyroid function, and also in cases of hyperthyroidism, hypothyroidism, Hashimoto's thyroiditis, thyroid enlargement, presence of a benign or malignant during pregnancy, and also to monitor the dosage of thyroid substitute drugs, in case of removal of this gland.

Usually this test is ordered for everyone over 40 years of age, even if there are no cases of thyroid disease in the family.

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