The euthyroid state is distinguished from hyperthyroidism and hypothyroidism using thyroid function testing. Available thyroid function tests include the TRH stimulation test, TSH test, TBG test, reverse T3, and Free thyroxine index (Nerenz, 2017). However, the best test to use is the TRH and TSH. TRH stimulation test is considered as the most sensitive and specific test for diagnosing thyroid disease because it measures the relationship between TRH and TSH discretion. On the other hand, the TSH test is the most crucial, best screening test and the most clinically sensitive assay for the detection of thyroid diseases. It is used to differentiate primary hypothyroidism from secondary hypothyroidism (Pirahanchi et al., 2022).

Reference Articles:

Nerenz, R. (2017). Thyroid function testing. AACC. Retrieved February 24, 2023, from https://www.aacc.org/science-and-research/clinical-chemistry-trainee-council

Pirahanchi , Y., Toro, F., & Jialal, I. (2022). Physiology, Thyroid Stimulating Hormone . National Library of Medicine. Retrieved February 24, 2023, from https://www.ncbi.nlm.nih.gov/books/NBK499850/

The thyroid set of thyroid function tests includes the TSH measurement by third-generation radioimmunometric assay, as well as the total and free thyroid hormones quantification. Free T4 (FT4)measurement is more valuable for diagnosing impaired thyroid function than total T4 because the free T4 fraction is not modified by factors that alter the thyroid hormone-binding proteins. Some laboratories also include free thriyodothyronine (FT3) as part of the thyroid function set. The usefulness of the set of thyroid function tests is to confirm whether the function of the thyroid gland and its relationship with the pituitary gland is normal or not, to investigate anomalies in the proteins transporting thyroid hormones, cases of tissue resistance to the thyroid hormones as well as tumors producing TSH. In the latter two cases, the TRH stimulation test is useful.

A thyroid function study can be done as a screening research study consisting of the quantification of TSH in the general population as a research tool to determine the prevalence of thyroid dysfunction or in case of the presence of risk factors of thyroid dysfunction, such as first degree relative with altered thyroid function, pregnancy, type 1 diabetes mellitus and subjects over 60 years of age, among others. In these cases, it is only necessary to measure serum TSH hormone, since it is the most sensitive test to assess alteration of thyroid function, and perform free T4 and TSH measurement if the clinical data are very suggestive of thyroid dysfunction, which avoids duplication of studies, and medical consultations. The treatment in case of suspected low thyroid function with normal thyroid profile tests is not justified since the symptoms are non-specific.