Thyroid function tests mainly include the following items.
1.Thyroxine secretion index
Total T3 (TT3), total T4 (TT4), free T3 (FT3), free T4 (FT4). T3 and T4 can be increased in pregnancy, birth control pills and estrogen therapy, but can be decreased in androgen and prednisone therapy, nephrotic syndrome, liver failure, and phenytoin sodium and phenytoin, thus limiting the accuracy of hyperthyroidism diagnosis.
However, FT3 and FT4 are not affected by the above mentioned factors and are commonly used for the diagnosis of hyperthyroidism and hypothyroidism. FT3 accounts for 0.3% of total T3 and FT4 accounts for 0.04% of total T4. Although the content is small, the biological activity is very high because only free T3 and T4 can enter the cells and play a physiological role.
FT3 is the first to rise in the early stage of hyperthyroidism or relapse, and is of great significance for the diagnosis of hyperthyroidism, while FT4 is also increased in hyperthyroidism, but is the first to decrease in hypothyroidism, and is better than FF3 for the diagnosis of hypothyroidism. anti-T3 (rT3): more than 95% is formed by deiodination of T4 during metabolism in peripheral tissues, and is not physiologically active, and is of little significance for the diagnosis of hyperthyroidism. it rises and falls simultaneously with T3, T4, FT3 and FT4 in hyperthyroidism. It only rises first in hyperthyroidism or early relapse of hyperthyroidism, and rT3 can rise independently in non-thyroidal diseases with normal T3, T4, FT3, FT4. T3 and T4 often rise simultaneously in hyperthyroidism, but there are exceptions.
(1) Elevated T3 only.
(1) T3-dominant hyperthyroidism: It is more common in iodine-deficient areas;
(2) T3 dominant hyperthyroidism: T4 has been normalized after hyperthyroidism treatment, but T3 continues not to drop or even increase, T3 (ng/mL)/T4 (mcg/mL) >30, this type has a high recurrence rate, and surgery is appropriate;
(3) Early hyperthyroidism or early relapse of hyperthyroidism. 4 When hypothyroidism is treated with thyroid tablets. The normal thyroid tissue is secreted according to the ratio of T31:T49, while the T3 in thyroid tablets is 11 and T4 is 12, the content is nearly equal, and the absorption rate of T3 is 95%, while T4 is only 40%, and the phenomenon of increased T3 will occur with long-term use. Therefore, it is now advocated to use levothyroxine sodium (eugenol), which can avoid T4 deficiency.
(2) T4 elevation only.
①T4 hyperthyroidism: it is related to eating too much iodine-containing food;
(2) Pseudo-T4 hyperthyroidism: common in heart attack, pulmonary heart disease, acute cerebrovascular disease, asthma, tumor, acute liver disease, etc., decreases as the primary disease improves;
(3) A few elderly hyperthyroidism, may only T4 high.
(3) T4 hypo.
①See in hypothyroidism ;
(2) Chronic heart, lung, liver and brain diseases.
2. Thyroid stimulating hormone (TSH)
TSH is produced in the pituitary gland and has the function of promoting the production of T3, T4, FT3 and FT4 by thyroid follicular cells. Its high and low levels are controlled by negative feedback from thyroxine, and its performance is opposite. When thyroxine is high, TSH is low; when thyroxine is low, TSH is high. Therefore, T3, T4, FT3 and FT4 are increased in hyperthyroidism, while TSH is decreased, while the opposite is true in hypothyroidism.
3.Thyroid immune index
Thyroid immunoglobulin (TG) (normal value <30%) and thyroid microglobulin (TM) (normal value <15%) are often measured. Both can also be increased in hyperthyroidism, but the titers are low and can return to normal within 6 months, so they are not significant for the diagnosis of hyperthyroidism. However, when the value of both tests is greater than 50%, it has diagnostic value for Hashimoto's hyperthyroidism.