※
In recent years, the prevalence of thyroid disease has shown a clear upward trend. Thyroid function measurements are also included in many health check-up packages. In the face of those high and low arrows on the nail test list, not only are the patients confused, but even some non-specialist doctors have difficulty in interpreting.
So, what is the meaning of the level of each indicator on the test sheet? How should it be reasonably interpreted in conjunction with the patient's medical history?
Serum thyroid hormones
Serum thyroid hormones, including TT3, TT4, FT3, FT4.
Thyroid hormone (TH) is an important indicator of thyroid function status, including thyroxine (T4) and triiodothyronine (T3), of which T4 is all secreted by the thyroid gland, only 20% of T3 is derived from the thyroid gland, and the remaining 80% is converted from T4 deiodine in peripheral tissues. The physiological activity of T3 and T4 is not the same, the physiological activity of the former is 5 times that of the latter, and the vast majority of T4 needs to be converted to T3 before it can exert physiological effects.
Thyroid hormones are available in both binding and free forms, with the vast majority (more than 99%) in the form of binding to plasma proteins (mainly thyroid-binding globulins, TBGs), and the remaining small amounts of thyroid hormones are in a free state. Conjugated thyroid hormones are the storage and transport forms of hormones, and free thyroid hormones (FT3, FT4) are the physiologically active forms of hormones. Theoretically, free thyroid hormones are not affected by changes in serum TBG concentrations and are more realistic reflections of thyroid function.
The results of the determination of serum total T3 (i.e., TT3, abbreviated T3) and total T4 (i.e., TT4, abbreviated T4) are affected by serum thyroid-binding protein (TBG), when TBG is elevated (such as pregnancy, estrogen therapy, taking contraceptives, etc.), T3 and T4 are elevated; when TBG is decreased (such as androgen and prednisone therapy, nephrotic syndrome, cirrhosis, etc.), T3 and T4 are reduced. Therefore, the real representative of thyroid function state is free T4 (that is, FT4) and free T3 (that is, FT3), but because the content of FT3 and FT4 in the blood is very small, the stability of the measurement results is not as good as T3 and T4, so it is not yet possible to completely replace TT3 and TT4 with FT3 and FT4.
In general, changes in FT3 and FT4 are consistent, elevated in hyperthyroidism and decreased at hypothyroidism, but at some stage the two may not be fully synchronized.
For example, in hyperthyroidism, serum FT3 elevation usually occurs earlier than FT4 elevation, so FT3 is more sensitive to the diagnosis of early hyperthyroidism and recurrence of hyperthyroidism; hypothyroidism is often the first to appear as a decrease in FT4, while FT3 can be normal (because TSH is elevated, it can promote the conversion of T4 to T3, so FT3 can be normal in early hypothyroidism patients), so FT4 is more sensitive to the diagnosis of early hypothyroidism.
In addition, "T3 hyperthyroidism" is mainly manifested as FT3 elevated, FT4 can not be increased; "T4 hyperthyroidism" is mainly manifested as FT4 elevated, FT3 can not be increased.
Thyroid-stimulating hormone (TSH)
Thyroid-stimulating hormone (TSH) is secreted by the pituitary gland, and its main roles are twofold: 1. stimulate the thyroid gland to secrete thyroid hormone (TH); 2. Promote the proliferation of thyroid tissue.
Thyroid-stimulating hormone (TSH) is regulated by the negative feedback of thyroid hormone (TH), which decreases TSH when TH is elevated and TSH rises when TH decreases. It should be noted that this "negative correlation" relationship between the two applies only to primary hyperthyroidism or hypothyroidism, but not to pituitary hyperthyroidism or hypothyroidism, which is usually a "positive correlation".
For example, patients with pituitary hyperthyroidism, because pituitary adenomas have the characteristics of spontaneous secretion and are not inhibited by feedback from thyroid hormones, are often manifested as FT3 and FT4 are elevated, and TSH is also elevated; patients with pituitary hypothyroidism are often TT4 and FT4 are reduced, while TSH is reduced or not elevated.
Thyroid-stimulating hormone (TSH) is the most sensitive indicator of thyroid function, in the early stage of thyroid dysfunction, TSH often precedes thyroid hormones (T3, T4) to give us a "warning", and if T4, T3 begin to change, then TSH changes are often very significant.
FT3 and FT4 are normal, and TSH is reduced, which is called "subclinical hyperthyroidism", indicating that "hyperthyroidism" will occur; FT3 and FT4 are normal, and TSH is elevated, called "subclinical hypothyroidism", indicating that "hypothyroidism" will occur.
TSH normal reference range: 0.4~4.0mIU/L.
Anti-T3 (rT3)
Anti-T3 (rT3) is mainly formed by deiodineization of T4 when metabolized in peripheral tissues, rT3 is similar in structure to T3, but does not have physiological activity. T4 can be converted to T3 and rT3 in peripheral tissues, and if rT3 generation increases, the conversion of T4 to T3 decreases accordingly, which can reduce the body's oxygen and energy consumption, which is a protective mechanism of the body.
In hyperthyroidism, rT3 rises and falls simultaneously with T3, T4, FT3, and FT4, but the elderly, patients with severe malnutrition and advanced cachexia nonthyroid disease (such as "low T3 syndrome") can also be elevated, so the specificity is not strong, and it is of little significance for the diagnosis of hyperthyroidism. In patients with "low T3 syndrome" (also known as "normal thyroid pathological syndrome", ESS), when T3, T4, FT3, ft4 are normal, rT3 can be independently elevated, but rT3 is decreased in patients with hypothyroidism, so rT3 is often used to distinguish hypothyroidism from "low T3 syndrome".
Normal reference range for rT3: 0.2 to 0.8nmol/L. A higher rT3/TT3 ratio indicates a more severe patient condition.
Thyroid antibody itself
Thyroid autoantibodies mainly include thyroid peroxidase antibodies (TPOAb) and thyroglobulin antibodies (TGAb), the determination of serum thyroid autoantibodies, mainly used to determine the cause of hypothyroidism, if the antibody level is significantly elevated, the high sign is "Hashimoto's thyroiditis (Hashimoto disease)".
It should be noted that the level of thyroid autoantibodies is not directly related to the severity of thyroid disease, and is usually not a target for clinical treatment, so there is no need to care too much about antibody levels, and there are currently no effective drugs to reduce antibodies.
TSH receptor antibody (TRAb)
TSH receptor antibodies (TRAbs) actually contain two components: irritating antibodies (TSAbs) and inhibitory antibodies (TSBAbs). Among them, TSAb is the pathogenic antibody of Graves disease, and its positive value plays an important role in the diagnosis and prognosis of Graves disease, and can also be used as a reference index for antithyroid drugs (ATD) to stop drugs. In addition, TSAb can also cause "neonatal hyperthyroidism" through the placenta, so it has a predictive effect on neonatal hyperthyroidism. However, due to the complex conditions of TSAb measurement, it has not been widely carried out in the clinic, and TRAb positive is often regarded as TSAb positive. TSBAb plays an important role in the pathogenesis of hypothyroidism.
Thyroglobulin (Tg)
Thyroglobulin (Tg) is secreted by thyroid follicular epithelial cells and stored in the follicular cavity of the thyroid gland. Normally, very small amounts of thyroglobulin can be released into the bloodstream (< 40 μg/L). Thyroglobulin in patients with total thyroid resection is often less than 5 μg/L or even completely undetectable, and if serum Tg levels rise again during follow-up, it indicates residual lesions or metastases.
Therefore, clinically, it is often by observing the dynamic changes of Tg to observe the efficacy of surgery and monitor whether differentiated thyroid cancer (DTC) recurs after surgery. If the postoperative blood Tg is elevated, it indicates tumor recurrence or metastasis, and if it is reduced to undetectable, it indicates a good prognosis.
It should be noted that the tumor tissue of medullary thyroid cancer is derived from thyroid C cells, not thyroid filter cell epithelial cells, so the serum Tg level of such cancer patients is not elevated or even decreased.
Source: Endocrine Channel of the Medical Community
Edited by: Yeah Reviewer: Xiao Ran