Pregnancy and Thyroid Hormones
Before we begin to explain the effects of thyroid hormone change and thyroid hormone release problems during pregnancy, let’s simply understand the normal metabolism of the thyroid.
Normal Thyroid Metabolism
The central stimulus begins with the stimulation of TSH (Thyroid Stimulating Hormone) secreted from the pituitary gland in the brain, stimulating the thyroid gland located in our neck. In order for the thyroid gland to secrete two main thyroid hormones called T3 and T4, TSH needs the regular iodine intake as well as the regular release. Although the main function of thyroid hormones is to regulate metabolism, problems with thyroid hormone release (under or over secretion), menstrual irregularity, menstruation, anemia, mental development problems in the fetus, problems in reproductive function and abortion in pregnancy, premature birth, preeclampsia, early placenta separation can watch with very negative situation.
Thyroid Physiology in Pregnancy
There are some physiological changes in thyroid functions during pregnancy:
1) The increase in estrogen concentration causes a 2-3 fold increase in protein that binds thyroid hormones, reducing the circulating free thyroid hormone concentration and leading to general stimulation in the thyroid hormone release axis. If the woman’s iodine intake is sufficient and there is no subclinical thyroid disease, the pregnant woman will meet this change easily without being affected by the TSH level.
However, if the woman has goitre, thyroid hormone levels are normal, TSH hormone is low, there is a history of thyroiditis before pregnancy or if iodine intake is insufficient, hypothyroid (thyroid hormone) insufficiency may develop.
2) We know that the TSH hormone stimulates the thyroid gland. The same receptors stimulate the HCG hormone that you know very well during pregnancy. 8-14. When the HCG hormone reaches its maximum between weeks, the release of thyroid hormones increases, causing a decrease in the level of TSH. For this reason, hyperthyroidism may develop with HCG increasing thyroid hormone release in the table called ‘hyperemesis gravidarum’ which progresses with severe nausea and vomiting in the early pregnancy period.
3) Due to some enzymes released from the placenta, the metabolism (use) of thyroid hormones is accelerating, especially in the second and third trimesters of pregnancy.
Synthesis and release of thyroid hormone of the infant (fetus) in the womb takes place in the 14th week of pregnancy. So what happens in the previous period of pregnancy? Of course, if the mother’s thyroid metabolism is normal, the mother meets the thyroid hormone need of the fetus like everything else. The placenta will allow small amounts of T3 and T4 to pass into the fetus. Here, thyroid hormone adequacy in the early period of pregnancy has an important place in fetal brain development. In a study published in the New England Journal of Medicine in 1999, IQ scores in school-age examinations of children born with mothers with untreated hypothyroidism, it has been shown that IQ rates are 7 points lower compared to the children of healthy women and women who receive thyroid hormone support from outside.
4) By the way, when we started talking, we talked about iodine deficiency. The urinary excretion of iodine increases during pregnancy. The ideal daily dose of iodine intake is 250 mg / day during pregnancy and breastfeeding. Outside of pregnancy, it is 150mg / day. If the intake is insufficient, the intake can be supported orally. Adequate intake should be ensured, especially in women living in areas where we know there is iodine deficiency.
Gebelikte tiroid hormon testleri yapılırken doğal fizyolojideki değişiklikler mutlaka gözetilmelidir. Tiroid bağlayan protein total T3 ve T4 değerlerini arttırdığı için kan düzeyleri de doğal olarak yüksek olacaktır (normalin 1.5 katı kadar) Gebelikte plasma düzeyi(kanın sıvı kısmı da arttığından normal değerleri değerlendirirken asla gebe olmayan kadındaki gibi davranılamaz. Bu iki değişkenden de etkilenmeyen ve bize rahat izlem olanağı veren serbest tiroid hormonlarıdır.
Gebelikte izlemde doğal beklenti gebelik haftası arttıkça TSH düzeyinin yükselmesi ve sebest T4 düzeyinin de azalmasıdır. TSH bizim erken gebelikte tarama için kulandığımız ilk basamak testtir ve sonrasına eğer gerekirse serbest T4, serbest T3 ve tiroid antikorları istenebilir. Ancak tiroid sorunlarında temel takip TSH ve serbest T4 ile yapılmalıdır. Gebelikte TSH’ın normal düzeyleri uluslararası platformda ‘The Endocrine Society’ tarafından ilk trimester için <2.5mU/ml ve 2.- 3. trimester için <3.0mU/ml olarak belirlenmiştir. Ancak son yayınlar eğer serbest tiroid hormonları normal ve tiroid antikorları negatifse tsh 4 değerinin altında kaldığı sürece takip edilebilir kararını bildirmiştir.
Gebelik planlayan ve bu yüzden doktora başvuran kadınlarda ilk basamak test serum TSH düzeyinin araştırılmasıdır. Erken gebelik döneminde ise serum TSH ve ST4 değerlerinin beraber değerlendirilmesi uygun olur. 16. gebelik haftasından sonra ise TSH takibinde tiroid hormon değişikliklerini izlemede çoğunlukla yeterli olacaktır.
Bu noktada yapılan testlerde sıkça gördüğünüz anti-TPO(anti tiroid peroksidaz) antikoru da önemli kabul edilmektedir. Zira üreme çağındaki kadında antiTPO nun yüksek bulunması; çocuk sahibi olamama, erken doğum ve düşük oranlarında artışa neden olabilmektedir. Gebe kadın populasyonunun %10’ unda atiTPO yüksektir. Bu kadınlarda tiroid fonksiyon testleri her ne kadar normal olursa olsun gebeliğin ilerleyen dönemlerinde takibe devam edilmeli ve subklinik hipotiroidi(tiroid hormon düzeyleri normal iken tsh’ın yüksek olması) bile olsa tiroid hormon replasmanı zamanında başlanmalıdır. Anti TPO su yüksek adımlarda %50 oranında doğum sonrası tiroidit olasılığı mutlaka hasta ve hekim tarafından akılda tutulmalıdır.
Pregnancy and Hypothyroidism
The most common thyroid problem during pregnancy is hypothyroidism. While this is the main reason for iodine deficient regions, autoimmune thyroid diseases are frequently observed in areas where iodine is normal (the body develops antibodies to its thyroid cells: autoimmune thyroiditis).
The frequency of hypothyroidism during pregnancy is about 5% of pregnant women. The rate of subclinical hypothyroidism (when thyroid hormones are normal, tsh is high) was determined as 2.5%.
Basically, the negative consequences of overt hypothyroidism that only subclinical hypothyroidism can affect can be listed as follows:
* premature birth
* low birth weight
* pre-eclampsia (pregnancy hypertension)
* placental detachment (early removal of the placenta in pregnancy)
* temporary breathing difficulty in newborn
* The formation of fetal neurons, fetal brain development and the related functions being affected
* influencing the neuropsychiatric and intellectual development of the newborn
Thyroid hormone replacement should be started dynamically in the obvious or subclinical hypothyroidism during pregnancy. Pregnant woman’s need is always higher than pre-pregnancy period.
If thyroid hormone replacement is currently applied in women with already diagnosed hypothyroidism, in 4-6th weeks, the dose may need to be increased by 30-50%. During pregnancy period, TSH and T4 should be monitored once a month, and TSH should be kept at the recommended intervals.
Pregnancy and Hyperthyroidism
The frequency of hyperthyroidism in pregnancy is less and it is given in publications ranging from 1% to 0.2%. While free thyroid hormones increase in hyperthyroidism, TSH is suppressed and its level decreases. TSH may also be suppressed in the physiological pregnancy changes and hyperemesis gravidarum (severe nausea and vomiting early in pregnancy), which we have to remember here. Physiological changes should not be confused with the hyperthyroid picture.
Hyperthyroidism during pregnancy is due to Graves’ disease in a range of 85%, and the patient’s goitre and blood tests often have high free thyroid hormones and suppressed (low) TSH as well as thyroid receptor antibodies.
There is no goiter and autoantibodies in the temporary thyrotoxicosis, which occurs with the thyroid stimulation of HCG, which is much less common and is the natural hormone of pregnancy.
Untreated hyperthyroidism in pregnancy;
* early separation of the placenta (placental detachment)
* low birth weight may develop in the fetus.
Hyperthyroidism may develop in the baby after birth due to autoantibodies passing through the mother.
If hyperthyroidism is not controlled during pregnancy, labour can lead to postpartum thyroid crisis in the woman.
Propylthiouracil should be preferred in the first 12 weeks in the treatment of hyperthyroidism during pregnancy. Taking the possible negative effect of propylthiouracil on the liver, methimazole is preferred after the first 12 weeks. Metimazole should not be used for the first 12 weeks as it may increase the rate of anomaly. If hyperthyroidism cannot be controlled with medications and surgery is mandatory, the best timing will be the second trimester. Radioactive iodine therapy cannot be used in any period of pregnancy.
In the so-called subclinical hyperthyroidism, TSH is suppressed, but the blood level of thyroid hormones is normal and treatment is not needed.
Routine screening across the country is controversial and ‘The Endocrine Society’ recommends only high-risk pregnant women thyroid screening. However, it is considered useful to take care of TSH while routine tests are arranged in early pregnancy.
If there is a problem with the thyroid, cooperation with an endocrinologist will be the most suitable option for diagnosis and treatment.