I. Overview
The thyroid gland is the largest endocrine gland in the body, and the thyroid hormones it secretes are indispensable for human growth and metabolism. Thyroid disorders are also the most common and frequent endocrine disorders at present. Especially in recent years, the incidence of thyroid disorders in China has been increasing due to ecological changes and the gradual spread of legislative iodine supplementation to prevent iodine deficiency diseases. How to better diagnose and treat thyroid diseases is an important topic for endocrinologists at present. This chapter briefly introduces the clinical significance of thyroid hormones, antibodies, receptors and other tests that are currently used in clinical practice at home and abroad for the reference of clinicians in their treatment work. The normal reference values vary from hospital to hospital and from instrument to instrument, and the values provided herein cannot be used as a standard.
II. Triiodothyronine (TT3)
Normal reference value: 1.05~3.45nmol/L (0.7~2.3μg/L)
Clinical significance: TT3 in blood is mostly converted from TT4 by deiodination in peripheral tissues, a few are secreted directly by the thyroid gland, and its biological activity is 5-10 times that of TT4. Its main physiological effect is to participate in the metabolism of various substances in the body and to promote growth and development. The level of blood concentration is mainly regulated by the feedback between the hypothalamic-pituitary-thyroid axis, which keeps the concentration of thyroid hormone in the blood in the normal range.
Elevated: hyperthyroidism (GD), T3 GD, hyperthyroid-binding globulin (TBG)emia (pregnancy, oral contraceptives, estrogen therapy, etc.), and thyroid hormone therapy overdose. TT3 is more valuable than TT4 in the diagnosis of GD.
In the course of antithyroid drug (ATD) therapy, because ATD only inhibits the synthesis of thyroid hormones, but not the secretion of thyroid hormones, it does not inhibit the conversion of TT4 to TT3, except for propylthiouracil (PTU). When evaluating the efficacy of GD, as long as the serum TT3 is still elevated, regardless of whether the TT4 is normal, GD should be considered as not yet controlled. If necessary, TRAb should be tested to determine the immune remission of GD.
Decrease: Hypothyroidism, reduced TBG binding, ATD treatment overdose, chronic renal failure and “low T3 syndrome” caused by various non-thyroidal diseases such as liver cirrhosis, myocardial infarction, malignancy, severe infection, diabetes, cerebrovascular accident and severe stress reaction.
Thyroxine (TT4)
Normal reference value: 58.5~170nmol/L (45~130μg/L)
Clinical significance: TT4 is a thyroid hormone synthesized and secreted by thyroid follicular epithelial cells and has the same physiological effects as TT3. In recent years, it has been suggested that TT4 is a pre-hormone of TT3 and is its reserve form. In general, it rises and falls in parallel with TT3, but its value is normal in T3 GD and increases singly in T4 GD. In subacute thyroiditis, elevated values may be seen due to thyroid follicle rupture. The “low T3 syndrome” can also be normal if it is not accompanied by “high T4emia”, but its low value is more significant than TT3 in the diagnosis of hypothyroidism.
Thyroglobulin antibody (TGAb)
Normal reference value: < 30%
Clinical significance: TGAb is an antibody produced by thyroglobulin (TG) in the follicular colloid of the thyroid gland, which enters the bloodstream and is a non-complement binding antibody. It can be significantly elevated in about 80% of patients with Hashimoto’s disease (HT) and Hashimoto’s GD. Elevations can also be seen in GD and primary hypothyroidism, but there is often an overlap between the elevations in Hashimoto’s GD and GD. Therefore, it is difficult to identify whether GD is complicated by HT, and it is necessary to combine clinical manifestations and perform needle histology or cytology if necessary. In addition, thyroid cancer and some autoimmune diseases such as rheumatoid arthritis and systemic lupus erythematosus can also be seen to be elevated. In normal people, especially women and the elderly, about 2-10% of positive results can be detected, which generally indicates a genetically susceptible individual.
V. Thyroid microsomal antibody (TMAb)
Normal reference value: < 15%
The clinical significance of TMAb is the same as that of TGAb, but the positive detection rate is higher than that of TGAb, and the combined test can increase the positive rate. The combination of the two tests can increase the positive rate. Recent studies have concluded that TMAb is the thyroid peroxidase antibody (see below for details).
Thyroid peroxidase antibody (TPOAb)
Normal reference value: < 20U/L
Clinical significance: TPOAb used to be called TMAb, but recent studies have confirmed that thyroid peroxidase (TPO) is a major component of microsomal antigens. Its clinical significance is the same as that of TGAb, which is commonly found in autoimmune thyroid disease (AITD). Currently, a highly purified TPOAb is used in immunoassays instead of the formerly non-purified TMAb, which has a higher sensitivity. It is mainly used clinically to monitor the effects of immunotherapy, to identify the likelihood of disease in individuals with a family history of thyroid disease and to predict the onset of postpartum thyroid dysfunction in pregnant women. It is also useful in solving clinical diagnostic challenges, such as abnormally high TSH levels accompanied by normal levels of FT4, when a positive TPOAb indicates subclinical hypothyroidism and early HT, and low levels of TPOAb in about 10% of asymptomatic patients, suggesting a susceptible population for AITD. Therefore, TPOAb has a higher clinical value than TGAb in the diagnosis of most AITD. Currently, the highly specific and sensitive TPOAb test has become the preferred method for detecting autoantibodies in the diagnosis and treatment of AITD. The authors conducted a two-way controlled study and found that the positive rate of TPOAb was 96%, TGAb was 76%, and TMAb was 84% in patients with initial diagnosis of GD, with the former being significantly higher than the latter. Among those who were positive for TPOAb, the positive detection rate of TGAb and TMAb was 86.66%, while among those who were positive for both TGAb and TMAb, the positive detection rate of TPOAb was 96.66%. Therefore, the authors believe that the future trend is to replace TGAb and TMAb with the highly sensitive TPOAb.(Methodological issues)
Note: When detecting thyroid autoantibodies, the serum should be separated immediately after specimen collection and stored at 4°C, or cryopreserved if you want to leave it for a longer period of time. The antibody potency of an unseparated serum specimen decreases rapidly at room temperature or 4°C.
Currently, many primary care physicians lack sufficient knowledge of the clinical significance of autoantibodies to the thyroid gland and therefore do not pay enough attention to their detection. This inevitably leads to the underdiagnosis of “Hashimoto’s disease” and “Hashimoto’s hyperthyroidism”, and also leads to treatment errors. In clinical practice, many scholars have found that the detection of thyroid autoantibodies is not only diagnostic but also important in guiding treatment and predicting immune remission in patients with first diagnosed thyroid disease.
VII. Thyroglobulin (TG)
Normal reference value: 15.85±4.4μg/L
Clinical significance: TG is a macromolecular protein synthesized by thyroid follicular epithelial cells and is the main component of the intrafollicular glial thyroid gland. Under normal conditions, TG circulates only in the thyroid lumen and does not spill into the blood. Only when the thyroid gland is diseased or physically damaged does TG enter the blood circulation. As early as the late 1960s, foreign scholars proposed the use of TG as a tumor marker for thyroid cancer, and by the mid-1970s TG was widely appreciated by clinicians. In some benign thyroid diseases (e.g. HT, thyroid adenoma and a few patients with GD), elevated serum TG can also be seen, so it is considered that TG measurement is non-specific for the diagnosis of thyroid disease. It is currently used only for observation of the efficacy and monitoring of recurrence of follicular thyroid carcinoma. If blood TG is elevated after surgery or RAI treatment, it indicates tumor recurrence or metastasis, and if it is reduced to an undetectable level, it indicates a good prognosis. Simultaneous testing with calcitonin (CT) is more meaningful. In medullary thyroid carcinoma, the serum TG level is reduced. Expert tip: TG testing should be performed only if TGAb is negative, because the presence of TGAb will seriously interfere with the TG results.
Note: Blood TG may be elevated to varying degrees after thyroid puncture or within 1 to 2 weeks after thyroid scan.
VIII. Anti-T3 (rT3)
Normal reference value: 0.54~1.46nmol/L
Clinical significance: rT3 is not biologically active in the human body, and its blood level is roughly synchronized with TT3 and TT4, and its clinical significance is the same as TT3 and TT4 in the diagnosis and monitoring of thyroid disease. In the diagnosis and monitoring of thyroid disorders, its clinical significance is the same as that of TT3 and TT4. In “low T3 syndrome”, its measured value is elevated and is now mostly used in the diagnosis of “low T3 syndrome”. In particular, the rT3/TT3 ratio is of great importance in determining the severity of various non-thyroidal diseases (NTI), and its ratio is significantly correlated with the disease. In a study of acute cerebrovascular disease (ACD), M. Vlatkovic et al. found that TT3 was significantly inversely correlated with rT3 in all patients, with a decrease in TT3 in blood paralleling an increase in rT3 and an increase in the rT3/TT3 ratio, which could be considered an indicator of a good prognosis for NTI when the ratio returned to normal. PTU is the most potent inhibitor of 5’deiodinase action and affects TT4 deiodination to TT3, therefore, changes in rT3 to TT4 values are directly related to PTU dose. In addition, dexamethasone also inhibits 5’deiodinase activity and suppresses rT3 metabolic profile, resulting in increased rT3 production and decreased TT3. The main degradation pathway of serum TT4 in neonates is to rT3, therefore, rT3 values can also be increased.
High-sensitivity thyrotropin (s-TSH)
Normal reference value: 0.3 to 4.4 mIu/L
Clinical significance: In the past, due to methodological problems, the sensitivity of TSH detection was limited and could only be used for the diagnosis of hypothyroidism. Since the establishment of the immunoradiometric analysis (IRMA) method in the mid-1980s, the sensitivity of TSH detection has been greatly improved, so that serum TSH level is considered to be the most sensitive indicator for the diagnosis of thyroid disease, and has shown its unique value for the diagnosis of GD and hypothyroidism, as well as the monitoring in the treatment of GD, especially in the diagnosis of GD, ultra-sensitive TSH can improve the sensitivity of diagnosis to the subclinical stage. The sensitivity of TSH is also better than FT3 and FT4 in predicting recurrence of GD, and there is a special relationship between TSH and FT4 in log/rectangular coordinates, where a small change in the concentration of FT4 will cause a strong response of TSH. At present, hospitals that are in a position to do so are basically using the IRMA method or chemiluminescence to detect TSH, instead of using the common methods with limited sensitivity.
Elevated: Primary hypothyroidism (after GD surgery or 131I treatment, etc.), subclinical hypothyroidism, iodine-deficient diaphoresis, hypothalamic GD, HT and postpartum thyroiditis, etc. Pituitary TSH-secreting adenomas secrete TSH autonomously from the pituitary gland, so the serum value can be abnormally elevated.
Decrease: GD, Hashimoto’s GD, subclinical GD, secondary hypothyroidism (pituitary and hypothalamic), Silhan’s disease, glucocorticoid overdose, prolactinoma, and thyroid hormone replacement therapy overdose. Therefore, if s-TSH measurement is still low, regardless of whether TT3, TT4, FT3 or FT4 is normal, it should be judged that GD has not been controlled and should not be stopped prematurely. For patients with subclinical GD with normal TT3, TT4, FT3, FT4 and reduced s-TSH, especially when s-TSH is <0.1mIU/L, regular monitoring of thyroid function is especially necessary to detect early clinical GD and give early treatment.
X. Free T3 (FT3)
Normal reference value: 2.8~8.5pmol/L
Clinical significance: Unlike TT3, it is the most sensitive and valuable indicator of thyroid function because it is not affected by the level of TBG and can correctly reflect the function of the thyroid gland, and it is also the update and development of TT3 testing technology. It is often used to determine thyroid function in pregnancy GD and low T3 syndrome. Combined with FT4 and s-TSH, it has become a new protocol for thyroid function testing. During thyroid hormone replacement therapy, FT3 is elevated before TT3; during ATD treatment, if FT3 is still elevated, GD should be judged as uncontrolled; if FT3 is normal and TT4 is below normal, GD should be judged as controlled and there is no hypothyroidism. Only when both FT3 and TT4 are below normal is ATD considered over-treated. (See summary section)
XI. Free T4 (FT4)
Normal reference value: 8.5 to 26.5 pmol/L
Clinical significance: Same as FT3, it can correctly reflect thyroid function and is not affected by TBG concentration, and its measured value is normal in T3 GD. FT4 can also be elevated due to the use of ethamivudine and can be elevated due to certain non-thyroidal diseases. (See summary section)
Thyroxine-binding globulin (TBG) (RIA)
Normal reference value: <20mg/L
Clinical significance: TBG is the main carrier protein of thyroxine in the blood circulation and is of great significance for the transport, storage, metabolism and relative constancy of free thyroid hormone. Changes in its concentration directly affect the levels of TT3 and TT4, making it more difficult to diagnose patients with abnormal thyroid function, and FT3 and FT4 should be tested at this time to improve accuracy.
Elevated: hypothyroidism, cirrhosis or liver damage, pregnancy, newborn, acute intermittent porphyria, collagen diseases, oral contraceptives and estrogen treatment can cause TBG to increase.
Decrease: Hyperthyroidism, various serious diseases, severe malnutrition, uncontrolled diabetes, malignancy, acute renal failure, nephrotic syndrome, active acromegaly, etc., as well as the use of large amounts of glucocorticoids, androgens, sodium phenytoin, salicylates and other drugs can reduce the concentration of TBG.
Thirteen, thyrotropin receptor antibody (TRAb) (RRA method)
Normal reference value: <9.0U/L
Clinical significance: TRAb is a class of specific immunoglobulins with heterogeneity, including two types of stimulatory antibodies (TSAb) and inhibitory antibodies (TBAb). The former is the main cause of the occurrence and development of GD, while the latter plays an important role in the pathogenesis of hypothyroidism. The extent of the effect on the thyroid gland and stimulation depends on the relative concentration and biological activity of the above two types of antibodies. It has been used abroad as a confirmatory index for the diagnosis of GD and is often used in immunological studies of GD and in the discussion of pathogenesis, and is also of some significance in the differentiation of GD from other thyroid diseases. During the treatment of GD, it is important to monitor whether TRAb turns negative or decreases to determine the efficacy and prognosis.
Some scholars have concluded that the pre-treatment TRAb level of GD patients is positively correlated with the treatment course. The results of foreign scholars suggest that the relapse rate of those who are still positive for TRAb after one year of ATD treatment is 90% within three years. Since the development of TRAb radioreceptor assay (RRA) in our department in 1996, we found that its positive detection rate for patients with primary diagnosis of GD was as high as 94.8%, which was close to that reported in foreign literature, while the positive detection rate for the same serum by ELISA was only 18.9%. Therefore, in cases of subclinical GD or difficult clinical confirmation, it is recommended to check TRAb by RRA method to help confirm the diagnosis. It is worth noting that the presence of high TRAb in the serum of pregnant women with GD should be highly alert to the possibility of transient GD in the newborn. This suggests that 3-6 months after 131I treatment is the peak period of immune function changes in patients, and immune remission is reached only after one and a half years.
Thyrotropin-releasing hormone (TRH)
Normal reference value: 13.8~165.7pmol/L (26.16±4.93pg/ml)
Clinical significance: The main physiological role of TRH is to stimulate the release of TSH from pituitary thyrotropic hormone cells, which can also promote the release of prolactin (PRL) and growth hormone (GH); in addition, large doses of TRH have an anti-shock effect. The detection of serum TRH is clinically important for checking the functional status of hypothalamic-pituitary-thyroid axis.
Elevated: primary hypothyroidism (both TRH and TSH are elevated), secondary hypothyroidism (pituitary hypothyroidism such as Silhan’s disease where TRH is elevated and TSH, TT3 and TT4 are decreased), late stage subacute thyroiditis, advanced breast cancer, spinal cord tumors, and the application of certain drugs such as stavudine may also show elevated TRH.
Decrease: Hypothalamic hypothyroidism (simultaneous decrease in TT3 and TT4 levels), hypothalamic dysfunction (often with other endocrine function changes), post-traumatic brain injury and the application of certain drugs such as barbiturate sedatives can all decrease TRH.
Since the concentration of TRH in peripheral blood is extremely small and is quickly inactivated by enzymes, it is difficult to detect and popularize, and TRH excitation test is now mostly used instead of TRH determination. The following is a brief description.
TRH excitation test: TRH can promote the synthesis and release of TSH. 20 minutes after intravenous injection of TRH, serum TSH rises, and this test can reflect the reserve function of TSH. The TSH increase (ΔTSH) was then calculated from the pre-injection basal TSH (BTSH) and post-excitation TSH (ATSH), and then the pituitary response to TRH was classified into five types according to the ΔTSH value as follows.
Normal response type: △TSH 5-25mIU/L, with peak time in 30 minutes;
Over-responsive: △TSH > 25mIU/L;
Low response type: △TSH < 5mIU/L;
Non-responsive: △TSH 0 mIU/L;
Delayed response type: the peak occurs after 30 minutes.
Clinical significance: TRH excitation test is the most sensitive indicator for the diagnosis of primary hypothyroidism, showing overreaction or delayed response. Pituitary hypothyroidism shows a weak response or no response. In hypothalamic hypothyroidism, the BTSH is lower than normal, and in patients with long duration of disease, the initial response to TRH excitation test may be weak, and the response may be delayed even after increasing the TRH dose. Analysis: If a patient does not secrete PRL even after a certain amount of TRH is given under the effect of dopamine blockers, functional or organic damage to the dopamine-producing tissues of the hypothalamus should be highly suspected and may be accompanied by pituitary lesions. The TRH excitation test is unresponsive in endocrine synesthesia with normal nail function. In addition, the TRH excitation test is important for the diagnosis of thyroid hormone resistance syndrome.
Caution: Estrogen, theophylline and excessive ATD treatment enhance the anterior pituitary stimulation response to TRH. Cortisol, thyroid preparations, and levodopa can inhibit the pituitary response to TRH, so the drug should be stopped for one month before the test.
XV. Summary
Due to the rapid development of thyroid endocrinology, the number of laboratory tests for thyroid function is increasing, which can sometimes cause confusion and unexplained contradictory results.
As thyroid disorders are autoimmune diseases, most patients have complex and variable conditions; it is more common for GD to be complicated by HT (Hashimoto hyperthyroidism), and it is not uncommon for hypothyroidism to be caused by HT. Therefore, it is recommended that thyroid function tests for first-time patients should include thyroid autoantibody TGA and TPOAb tests as routine items, in addition to s-TSH, TT3 or FT3, TT4 or FT4, to avoid therapeutic errors. Recently, it has also been suggested that the best tests for screening for thyroid disease are s-TSH with TPOAb.
In addition, clinical testing often encounters cases where s-TSH and FT4 test results are inconsistent, such as abnormal s-TSH and normal FT4 or abnormal FT4 and normal s-TSH. These phenomena are commonly associated with subclinical thyroid disease and the presence of thyroid hormone antibodies (TH-Ab) in the blood.
In the course of medical treatment of thyroid disease, regular and selective examination of thyroid function, especially s-TSH, TPOAb and TRAb, can help to adjust the dose of medication and determine the course of treatment, efficacy and prognosis.
In addition, in recent years, thyroid hormone abnormalities in patients with non-thyroidal diseases (NTI) have attracted widespread clinical attention, and these patients do not have clinical manifestations of thyroid disease, so they are called “Euthyroid Sick Syndrome” (ESS). The most common types of ESS are “low T3 syndrome”, “low T3 and T4 syndrome” and “high T4 syndrome”, especially “low T3 syndrome”. The “low T3 syndrome” is the most common, accounting for 70% of the critically ill inpatients. The “low T3 syndrome” in diabetic patients is also very common, and our findings suggest that low TT3 levels in diabetic patients are often closely related to the severity of the disease, but not to the type of diabetes. Therefore, dynamic detection of TT3, TT4, rT3 and s-TSH is important to observe the outcome and determine the prognosis of critically ill patients.