Serum TSH measurement is used to assess possible thyroid dysfunction and the detection of mild (subclinical) forms as well as the apparent cases of primary hypo- and hyperthyroidism, to monitor patients with thyroid replacement therapy, confirmation of TSH suppression in patients with suppression therapy of thyroid cancer and for prediction of response following stimulation with thyrotropin-releasing hormone.
When thyroid hormone levels drop in the bloodstream or the body is exposed to physical or psychological stress, the hypothalamus is stimulated and releases the thyrotropin-releasing hormone (TRH). TRH in turn stimulates the production of thyroid-stimulating hormone (TSH) from the anterior pituitary lobe. TSH then stimulates the production and release of triiodothyronine (T3) and thyroxine (T4) from the thyroid gland. As T3 and T4 levels rise in the blood, the pituitary gland is stimulated to reduce TSH production through a negative feedback mechanism.
The release of TSH follows a daily pattern, with the highest observed late in the evening and the smallest observed in the middle of the day. The measurement of TSH concentration together with FT4 is used to diagnose primary and secondary hypothyroidism. Primary diseases are related to the target organ (the thyroid gland), while secondary diseases are related to pituitary gland abnormalities. For example, in primary hypothyroidism, the thyroid gland malfunctions and produces very low levels of free T4 in the blood. The anterior pituitary lobe detects low levels of T4 in serum and thereby increases TSH release. In this way, an attempt is made to stimulate the thyroid gland to increase T3 and T4 production. As the thyroid gland does not respond to this stimulation, TSH levels are constantly increasing.
Screening for thyrotropin hormone in the serum is a hypersensitive indicator that has largely replaced all other tests used to test and diagnose hypothyroidism and to monitor treatment. Third-generation TSH tests are considered the most appropriate for initial screening when thyroid disorders are suspected. If thyrotropin hormone determination is normal, no further tests are needed. If TSH determination is out of bounds, thyroxine (T4) should also be checked. Patients who have elevated TSH levels but normal levels of free thyroxine (FT4) are considered to have subclinical hypothyroidism. Subclinical hyperthyroidism is diagnosed in vitro when the serum thyrotropin hormone level is low while both free thyroxine and triiodothyronine (T3) are at normal levels.
Thyroid-stimulating hormone measurement is also used to monitor the patient's response to the treatment of thyroid disorders.
Possible Interpretations of Pathological Values
- Increase: Addison's disease (primary), anti-TSH antibodies, eclampsia, euthyroid goiter (with abnormal enzymes), fasting status, goiter (due to iodine deficiency), hyperpituitarism, hypothyroidism (primary), hypothermia, pituitary adenoma (secreting thyroid-stimulating hormone), postoperative (partial thyroidectomy), preeclampsia, acute psychiatric illness, after treatment with radioactive iodine, thyroiditis. Medications: Amiodarone, benserazide, clomiphene, iopanoic acid, lithium, methimazole, metoclopramide, morphine, propylthiouracil, radiocontrast agents, thyrotropin-releasing hormone
- Decrease: Hashimoto's thyroiditis, hyperthyroidism, hypothyroidism (secondary, tertiary) (sometimes), organic cerebral syndrome. Medications: Aspirin, heparin, ketoconazole, dopamine, glucocorticoids, octreotide, thyroid hormones
Laboratory test results are the most important parameter for the diagnosis and monitoring of all pathological conditions. 70%-80% of diagnostic decisions are based on laboratory tests. The correct interpretation of laboratory results allows a doctor to distinguish "healthy" from "diseased".
Laboratory test results should not be interpreted from the numerical result of a single analysis. Test results should be interpreted in relation to each individual case and family history, clinical findings, and the results of other laboratory tests and information. Your personal physician should explain the importance of your test results.
At Diagnostiki Athinon we answer any questions you may have about the test you perform in our laboratory and we contact your doctor to get the best possible medical care.