The Ill Effects of Hypothyroidism and Pregnancy
Hypothyroidism and pregnancy are not mutually exclusive! However, if you’re pregnant, or planning to get pregnant, knowing a bit about how your thyroid functions in relation to pregnancy is a good idea because even women with perfectly normal thyroid function can experience thyroid problems either during or after pregnancy.
Normal Hormonal Changes During Pregnancy
Pregnancy causes a great many hormonal and physiological changes throughout the body. Some of these affect thyroid function. When you’re first pregnant, a hormone called human chorionic gonadotropin (hGC) produced by the placenta, helps the corpus luteum to develop. The corpus luteum is responsible for producing progesterone to maintain the pregnancy and develop the endometrium (lining of the uterus) so it can support a growing fetus.
Human Chorionic Gonadotropin
Human chorionic gonadotropin can also interact with TSH receptors on thyroid tissue cells to mildly increase production of thyroid hormones, notably during the first trimester when levels of hGC are highest. This results in a mild (subclinical) form of hyperthyroidism (excess thyroid hormones). When blood levels of thyroid hormones rise, the pituitary reduces production of TSH hormone.
Scientists are not sure why it’s necessary for hGC to get involved with thyroid hormone production like this but it may have something to do with the fetus’s requirement for maternal thyroid hormones during the first trimester. Once hGC levels drop during the 2nd and 3rd trimester TSH production should return to normal.
Interestingly studies have also shown that women who suffer from severe morning sickness usually have higher levels of hGC than women who don’t.
Estrogen is another hormone that affects thyroid hormones during pregnancy. It increases the level of thyroid binding proteins in blood serum. These are the proteins that carry the thyroid hormones around the body. Whilst the hormone molecules are bonded to the protein they’re inactive. To become active (able to be used by the body) they must be ‘freed’ from the proteins.
When you see the terms ‘free T4’ and ‘free T3’ it is referring to hormone molecules that have been separated from their carrier proteins and are available for the body to use. Therefore, when you’re pregnant, the amount of thyroid hormone in your blood serum increases. However, most of it remains bonded to the transporter proteins so your actual free hormone levels remain normal.
Your thyroid gland will also enlarge slightly during pregnancy but should not be visible. Around 10 – 15% is normal. If you do develop a noticeably large thyroid (goiter) your doctor will probably arrange to test your thyroid function.
At around 12 weeks into gestation the fetal thyroid glands develop but they don’t start producing their own hormones until around 18 – 20 weeks. During that time it relies on maternal thyroid hormones. Once fetal thyroid hormone begins to happen, the fetus requires iodine which it obtains from the mother.
Therefore your dietary requirements for iodine will increase to not only support your own thyroid hormone production but also that of your unborn child. Normal recommended dietary iodine is 150 micrograms and this usually increases to 200 micrograms during pregnancy. In developed countries iodine deficiencies are uncommon thanks to products like iodized salt and iodine fortified foods.
Thyroid hormones are vital for the normal development of the fetal central nervous system and brain. Shortfalls in these hormones, particularly in the first trimester before fetal thyroid hormone production begins, can cause neurological, cognitive and developmental problems in the baby. Whilst this is more commonly associated with severe untreated maternal hypothyroidism caused by iodine deficiency, there is increasing evidence to suggest that even mild untreated hypothyroidism during pregnancy can lead to mild neurological abnormalities in the baby.
A new born screening test is now done shortly after birth in developed countries to pick up problems like congenital hypothyroidism. If thyroid abnormalities are present, appropriate treatment as required is started as soon as possible.
Hypothyroidism and Pregnancy – The Risks
Generally speaking, if you have an under active thyroid you should be fine so long as you keep taking your medications. You should nevertheless have a TSH test when your pregnancy is confirmed to ensure your current dosage is sufficient. It’s frequently the case that dosages need to be increased during pregnancy to account for the fact that you are supplying hormones for 2 people. You will probably need to be tested every 6 weeks or so during the first half of your pregnancy to make sure your dose is still high enough, and at least once during the last trimester.
The biggest problems with hypothyroidism and pregnancy lie with undiagnosed and untreated hypothyroidism, particularly severe hypothyroidism. Under those circumstances, hypothyroidism can cause:
- late pregnancy rise in blood pressure known as pre-eclampsia
- abnormalities in the placenta
- still born baby
- low birth weight of your baby
- transient congenital hypothyroidism in the baby
- postpartum hemorrhage
- and, in rare cases, congestive heart failure
Hypothyroidism and Pregnancy – The Diagnosis
As with many things to do with subclinical hypothyroidism, there are differing opinions on whether or not all women should be screened for thyroid problems during pregnancy. Part of this is due to the fact that current thyroid function tests can be easily misinterpreted and incorrect diagnoses made.
Some doctors believe it’s important to check TSH levels either just before pregnancy or upon confirmation of pregnancy. Others may not see the need unless the woman is in one of the high-risk groups (family history of thyroid problems, has a goiter or has had previous treatment for hyperthyroidism).
Diagnosing hypothyroidism and pregnancy is a case of having blood tests done to check thyroid hormone levels. At the same time, the doctor will be looking for antibodies to find out the underlying cause – Hashimoto’s disease, iodine deficiency etc.
If you are diagnosed with hypothyroidism, you will be put on a course of replacement thyroid hormones, typically levothyroxine, a synthetic hormone that replaces T4. This is the safest type of hormone for your unborn baby until he or she can begin making their own.
If you have any concerns at all about your hypothyroidism and pregnancy, it’s always best to speak to your doctor.