What is meant by thyroid function tests?

Posted by Resident Doctor Trinh Ngoc Anh - Department of General Internal Medicine - Vinmec Times City International Hospital

As a very important endocrine gland, the thyroid gland is involved in the regulation of metabolism of most organs in the body. In clinical practice, to aid in the diagnosis and monitoring of thyroid disease, doctors often order thyroid function tests.

TSH is a pituitary hormone that sends signals to the thyroid gland and participates in the regulation of thyroid hormone secretion. If the pituitary gland detects that there is less thyroid hormone in the blood, it will produce more TSH, prompting the thyroid gland to produce more hormone. On the other hand, if the pituitary gland detects that it has too much thyroid hormone, it will reduce its production of TSH, which in turn leads to a decrease in thyroid hormone production as well.
Normal TSH: 0.4 - 5 mIU/L.
Indications for thyroid-stimulating hormone (TSH) test:
To diagnose thyroid function conditions (hypothyroidism, hyperthyroidism) and monitor response to treatment; Distinguish whether the source of hypothyroidism is glandular or extra-glandular. Interpretation of results:
Changes in TSH values ​​can act as an early warning system for thyroid function, these changes occur before thyroid hormones are affected. If the TSH level is high, the thyroid gland is not making enough thyroid hormone (hypothyroidism due to a disease in the thyroid gland). A low TSH level usually indicates that the thyroid gland is producing too much thyroid hormone (hyperthyroidism due to thyroid disease). Sometimes low TSH can also be the result of an abnormality in the pituitary gland, which prevents it from making enough TSH to stimulate the thyroid gland (hypothyroidism due to damage to the hypothalamus or pituitary gland).

It is the main form of thyroid hormone in the circulating blood. In the blood, T4 exists in 2 forms:
Free T4 (FT4): accounted for 0.02% and is the biologically active form; T4 bound to protein: accounts for 99.98% and is the biologically inactive form. Indications: To diagnose conditions of thyroid function (hypothyroidism, hyperthyroidism), especially in combination with TSH.
Interpret results:
High TSH and low FT4: Reflecting primary hypothyroidism due to thyroid disease. Low TSH and low FT4: Reflects hypothyroidism due to a problem with the pituitary gland. Low TSH and elevated FT4: Reflecting hyperthyroidism due to thyroid disease or overdosage of thyroid hormone. Normal index:
Total T4: 45.2 -110 ng/ml; Free T4 (FT4): 8.6 – 17.9 ng/l.

Is a form of thyroid hormone that acts directly at cells and is more active than T4. The majority of T3 is generated in peripheral tissues beyond the thyroid gland from T4.
In blood T3 exists in 2 forms:
Free T3 (FT3): accounted for 0.2% and is biologically active form; T3 bound to protein: makes up 99.8% and is biologically inactive. Normal index:
Total T3: 0.6 -1.84 μg/l; Free T3 (FT3): 2.31 – 4.29 ng/l. Indication: When the patient has symptoms of hyperthyroidism but the free T4 (FT4) result is normal or borderline. The Triiodothyronine (T3) test helps evaluate thyroid function and diagnose cases of T3-induced hyperthyroidism.
Interpret results:
Diagnose hyperthyroidism or determine the severity of hyperthyroidism: Patients with hyperthyroidism will have elevated T3 levels. In some people with low TSH, only T3 is elevated and FT4 is normal. The T3 test is rarely useful in patients with hypothyroidism, as it is the last test to be abnormal. Patients may have severe hypothyroidism with high TSH and low FT4, but have a normal T3. Currently, the Laboratory - Vinmec International General Hospital is one of the leading prestigious addresses in the country, trusted by a large number of patients for medical examination and treatment. Thanks to the use of modern machines and equipment to examine and analyze patient samples, it has provided accurate information, helping doctors to assess the overall health of the person being tested. Early detection of disease and timely treatment plan. In particular, with a space designed according to 5-star hotel standards, Vinmec ensures to bring patients the most comfort, friendliness and peace of mind when examining and treating thyroid diseases at the hospital.