What hormone is TSH?
TSH (thyrotropin) is a hormone secreted by the pituitary gland. (The pituitary gland is an important endocrine organ in the brain that synthesizes and secretes many hormones that facilitate the work of target organs)
TSH is an important pituitary hormone that maintains normal thyroid function in the body. It has a regulatory effect on the thyroid gland, promotes the proliferation and growth of thyroid cells, and facilitates the synthesis and secretion of thyroid hormones.
In layman’s terms, the pituitary gland is the headquarters and the thyroid gland is the department. The pituitary gland gives instructions to the thyroid gland to make it work. The thyroid gland in turn responds to the pituitary gland from time to time.
What are the major thyroid hormones?
Thyroid hormones are hormones that are synthesized and secreted by the thyroid gland. The biologically active thyroid hormones include triiodothyronine (T3) and tetraiodothyronine (T4, also called thyroxine). Thyroxine (T4) is secreted entirely by the thyroid gland, with T320% coming directly from the thyroid and 80% being converted in peripheral tissues by deiodination of T4. T3 is highly biologically active.
Usually we want to clarify the function of the thyroid gland, T4 (thyroxine) is more important.
How does TSH interrelate with T3 and T4?
TSH and thyroid hormones (T4 T3) interact to regulate thyroid function in the body. there is a feedback relationship between TSH and thyroid hormones. t3 T4 is the most important physiological suppressor of normal TSH secretion.
In the case of primary thyroid disease, an increase in circulating thyroid hormone levels (hyperthyroidism) will inhibit TSH secretion by the pituitary gland, resulting in an increase in T4 and T3 levels and a decrease in TSH. Conversely, if circulating thyroid hormone levels are insufficient (hypothyroidism), the pituitary gland will strive to secrete TSH, stimulating the thyroid gland to synthesize and secrete enough thyroid hormone to maintain the body’s needs. This is reflected in low T4 T3 levels and elevated TSH.
Of these hormones, TSH is the most responsive, and can change before T4 T3 shows significant changes. If T4 T3 has changed, the change in TSH is usually more significant.
How do thyroid hormones normally change during pregnancy?
There are many important physiological and hormonal level changes that occur in the body during normal pregnancy that can affect thyroid function, so reports of thyroid function needed during pregnancy need to be interpreted in the context of the pregnancy.
Two of the most significant hormones are human chorionic gonadotropin hCG and estrogen, and changes in these two hormones can cause changes in thyroid hormone levels.
1. The placenta secretes human chorionic gonadotropin (hCG) in the early stages of pregnancy, and usually when we test for pregnancy, we test for this hormone through urine and blood. hCG levels are high, and the pregnancy test is positive, suggesting pregnancy. hCG usually peaks at 8-10 weeks, with a concentration of 30,000-100,000 IU/L. hCG is partially similar in structure to TSH, and also has stimulating effect on the thyroid gland. The increase of thyroid hormone will suppress the secretion of TSH, so that the serum TSH level of pregnant women will be reduced by 20-30% in the early pregnancy. the lower limit of TSH level will be decreased by 0.4 mIU/L on average compared with non-pregnant women, and 20% will be decreased to less than 0.1 mIU/L. Generally, the lowest point of TSH decrease is at 10-12 weeks of pregnancy.
2. In early pregnancy, free T4 levels will increase by 10-15% compared to pregnancy because of hCG stimulation of the thyroid gland.
3. Estrogen levels also increase during pregnancy, generally starting to increase at 6-8 weeks of gestation, peaking at 20 weeks of gestation and continuing until secretion, increasing 2-3 times from the base. Elevated estrogen causes an increase in the production of thyroxine-binding globulin. The vast majority of thyroid hormones in the blood are bound to thyroid binding proteins, the result of which is an increase in maternal total T3 ,total T4 levels during pregnancy that does not reflect the exact levels of circulating thyroid hormones during pregnancy. Free T3 and free T4 during pregnancy are more accurate representations of thyroid function.
Why do we pay so much attention to thyroid function during pregnancy?
Until the 12th week of pregnancy, the fetus is completely dependent on the mother to provide the thyroid hormones he/she needs. At the end of the first trimester (after 12 weeks), the fetus’ thyroid gland becomes capable of producing thyroid hormones on its own, but still requires sufficient amounts of iodine from the mother to maintain its need for thyroid hormone synthesis. Thyroid hormone is an important hormone for growth and development, and has a role in the development of the brain, bones, and reproductive organs.
Clinical hypothyroidism during pregnancy impairs the neurointellectual development of the offspring. Increases the risk of preterm birth, miscarriage, low birth weight, stillbirth and gestational hypertension.
Poor control of thyrotoxicosis (excess thyroid hormones in the blood) is associated with miscarriage, gestational hypertension, preterm delivery, low birth weight babies, intrauterine growth restriction, stillbirth (death of the fetus during delivery), thyroid crisis and maternal congestive heart failure.
What is a good range for thyrotropin control during pregnancy?
American Thyroid Association guidelines.
The 2011 ATA guidelines (American Thyroid Association) were the first to suggest specific TSH reference values for all three trimesters of pregnancy.
T1 stage (early pregnancy to 12 weeks) 0.1 to 2.5 mlU/L
T2 stage (13 to 27 weeks of gestation) 0.2-3.0 mlU/L
T3 stage (late pregnancy 28 ~ ) 0.3-3.0mlU/L
How is clinical hypothyroidism in pregnancy diagnosed?
Serum TSH > upper reference value in pregnancy (97.5th), serum FT4 < lower reference value in pregnancy (2.5th).
How to diagnose subclinical hypothyroidism in pregnancy?
Serum TSH > upper limit of reference value in pregnancy (97.5 th), serum FT4 within the reference value range 2.5 th-97.5 th).
Our experts recommend that
According to several clinical studies in China at present, showing the general elevation of serum TSH levels in our population
TSH>2.5 mlU/L is not recommended as a diagnostic criterion for subclinical hypothyroidism during pregnancy in China.
If your institution is not in a position to establish its own pregnancy-specific reference ranges, it is recommended to use the reference ranges provided by our guidelines. However, attention should be paid to the matching of the assay reagents.
Does subclinical hypothyroidism in pregnancy require treatment?
For pregnant women with subclinical hypothyroidism who are positive for TPOAb (thyroid peroxidase antibody). Treatment with levothyroxine tablets is recommended.
In pregnant women with TPOAb-negative subclinical hypothyroidism. The Chinese guidelines for the management of thyroid disorders in pregnancy and postpartum recommend neither against nor recommend levothyroxine tablet therapy.
What are the treatment goals for clinical hypothyroidism in pregnancy?
T1 stage (early pregnancy) 0.1 to 2.5 mlU/L
T2 (mid-pregnancy) 0.2-3.0 mlU/L
Stage T3 (late pregnancy) 0.3-3.0mlU/L
Once clinical hypothyroidism is identified, treatment should be started immediately to rapidly achieve the above targets.
The treatment goals for subclinical hypothyroidism in pregnancy are the same as those for clinical hypothyroidism.
For each pregnant mother, the specific situation needs to be analyzed on a case-by-case basis. For pregnant women who are difficult to judge whether to use medication at the first time, dynamic observation of TSH and FT4 FT3 changes during pregnancy to see the trend is also a basis for us to judge whether to give medication intervention.