How to read the examination indicators of hyperthyroidism

We know that patients with hyperthyroidism may have various physical manifestations, severe form being a more prominent condition. In addition, in daily life, patients with hyperthyroidism are always prone to hunger. Even though they eat a lot, they are still very thin and their metabolism is accelerated. If the above situation occurs, you must go to the hospital for examination in time. Through the five thyroid function tests, we can preliminarily determine whether you have hyperthyroidism. So, how do you look at the examination indicators for hyperthyroidism?

1. Free T3, free T4. ：It is not affected by thyroid hormone binding protein and directly reflects the state of thyroid function. Its sensitivity and specificity are significantly higher than total T3 and total T4, because only free hormone concentration can accurately reflect thyroid function, especially in cases where TBG concentration changes significantly during pregnancy, estrogen therapy, familial TGB increase or deficiency, etc. It is more important. It is believed that the combined determination of FT3, FT4 and high-sensitivity TSH is the preferred option and first-line indicator for thyroid function assessment. Elevated levels of both are seen in hyperthyroidism; decreased levels are seen in hypothyroidism, hypopituitarism, and severe systemic diseases.

2. Serum total thyroxine TT4: It is the most basic screening test for determining thyroid function. TT4 includes the total amount of protein-bound proteins and is affected by changes in the amount and binding capacity of TBG and other binding proteins. Elevated TBG is common in hyperestrogenic states, such as pregnancy or patients treated with estrogen, and women taking oral contraceptives. Patients with congenitally high TBG and patients with familial abnormal hyperthyroxinemia have elevated TT4. TT4 is reduced in patients with hypoglycemia (such as cirrhosis and kidney disease), those taking drugs such as diazepam and testosterone, and those with congenitally low TBG. At this time, the physiologically active FT4 and FT3 should be measured to effectively evaluate thyroid function.

3. Serum total triiodothyronine TT3: The amount of T3 in serum bound to protein is more than 99.5%, so TT3 is also affected by the amount of TBG, and the changes in TT3 concentration are often parallel to TT4.

Increased serum TT3 and TT4 concentrations are mainly seen in hyperthyroidism. Together with FT3 and FT4, they can be used in the diagnosis, disease assessment, and efficacy monitoring of hyperthyroidism and hypothyroidism. However, in the early stages of hyperthyroidism and the early stages of relapse, TT3 generally rises rapidly, about 4 times the normal level; TT4 rises slowly, only 2.5 times the normal level. Therefore, TT3 is a sensitive indicator for observing the efficacy of early Hashimoto's disease and recurrence after medication discontinuation. Elevated TT3 and TT4 can also be seen in active hepatitis and during pregnancy. Decreased T3 and T4 can be seen in hypothyroidism. In hypothyroidism, TT4 or FT4 decreases earlier than TT3 or FT3. Decreased serum total T3 or FT3 is only seen in the late stages of the disease or in patients with severe illness. In addition, a decrease in both levels can be seen in conditions such as hypopituitarism, malnutrition, nephrotic syndrome, renal failure, and severe systemic diseases.

4. TSH measurement TSH is secreted by the anterior pituitary gland and is composed of α and β subunits. Its physiological function is to stimulate the development of the thyroid gland and synthesize and secrete thyroid hormones. The secretion of TSH is influenced by the excitatory effect of hypothalamic thyroid-stimulating hormone, the inhibitory effect of somatostatin, and the negative feedback regulation of peripheral thyroid hormone levels. A 15%-20% change in thyroid hormone levels can cause a 50%-100% change in TSH levels. TSH is not affected by TBG concentration and is less susceptible to interference from non-thyroid diseases that can affect T3 and T4. When thyroid function changes, TSH changes more rapidly and significantly than T3 and T4. Therefore, TSH in the blood is a sensitive test that reflects the function of the hypothalamus-pituitary-thyroid axis, especially for the diagnosis of subclinical hyperthyroidism and subclinical hypothyroidism.

Increased TSH may be seen in primary hypothyroidism, thyroid hormone resistance syndrome, ectopic TSH syndrome, TSH-secreting tumors, and when using dopamine antagonists and iodine-containing drugs.

Decreased TSH may be seen in hyperthyroidism, subclinical hyperthyroidism, PRL tumor, CUSHING disease, acromegaly, and excessive use of glucocorticoids and antithyroid drugs.

The earliest manifestation of primary hypothyroidism is elevated TSH. If TSH is elevated while T3 and T4 are normal, it may be subclinical hypothyroidism. Testing TSH in umbilical cord blood, neonatal blood or amniotic fluid at the 22nd week of pregnancy can help diagnose fetal or neonatal hypothyroidism.