Pregnancy can cause hypothyroidism, and it does so more often than most people realize. An estimated 2 to 3 percent of women are hypothyroid during pregnancy, with the majority of those cases being subclinical, meaning the thyroid is underperforming but not yet producing obvious symptoms. Pregnancy dramatically increases the workload on your thyroid gland, and for some women, the gland simply can’t keep up.
Why Pregnancy Strains the Thyroid
From the moment of conception, two hormones begin reshaping how your thyroid operates. Human chorionic gonadotropin (hCG), the same hormone detected by pregnancy tests, can weakly stimulate the thyroid. This is why TSH levels often dip in the first trimester. At the same time, rising estrogen increases the amount of proteins in your blood that bind to thyroid hormone, effectively pulling free hormone out of circulation. Your thyroid has to produce more hormone just to maintain normal levels.
The net result is that your thyroid needs to ramp up output by roughly 25 to 50 percent during pregnancy. If your thyroid was already borderline or slightly sluggish before conception, pregnancy can tip it into frank hypothyroidism. Women who were already taking thyroid medication before becoming pregnant typically need a dose increase in the first trimester, on average about 17 percent higher than their preconception dose.
Who Is Most at Risk
The biggest predictor of developing hypothyroidism during pregnancy is the presence of thyroid peroxidase (TPO) antibodies. These are markers of autoimmune activity against the thyroid gland. Women who carry TPO antibodies face a significantly higher risk of thyroid problems during and after pregnancy, even if their thyroid function tested normal beforehand. TPO antibodies are also independently associated with a nearly fourfold increase in miscarriage risk and roughly double the risk of preterm birth.
Other factors that raise your risk include a personal or family history of thyroid disease, type 1 diabetes or other autoimmune conditions, age over 30, morbid obesity, a history of pregnancy loss or infertility, and living in an area with low iodine intake. Having had two or more prior pregnancies also increases your likelihood of thyroid dysfunction.
How It Gets Diagnosed
Thyroid function in pregnancy is measured the same way as outside of pregnancy, through a blood test for TSH. But the normal ranges shift during pregnancy. TSH naturally drops in the first trimester due to hCG stimulation, then gradually rises through the second and third trimesters. First-trimester TSH typically falls between roughly 0.02 and 3.78, second-trimester between 0.47 and 3.89, and third-trimester between 0.55 and 4.91. These ranges can vary by population, which is why guidelines now recommend using region-specific reference values when available.
There is ongoing debate about whether all pregnant women should be screened for thyroid problems. The American Thyroid Association currently recommends targeted screening for women with known risk factors rather than universal testing, citing insufficient evidence to justify screening everyone. However, some researchers argue that universal screening in early pregnancy would catch cases that targeted screening misses. In practice, many clinicians now test TSH at the first prenatal visit, especially if any risk factors are present.
Risks of Untreated Hypothyroidism in Pregnancy
During the first trimester, your baby’s brain depends entirely on your thyroid hormone supply, since the fetal thyroid doesn’t start functioning until around 12 weeks. Even mild thyroid hormone insufficiency during this window has been linked to neurodevelopmental effects in children. A meta-analysis found that subclinical hypothyroidism during pregnancy was associated with more than double the odds of intellectual disability markers in offspring. Low free thyroid hormone levels carried a 63 percent increased risk of similar impairment.
For the mother, untreated hypothyroidism raises the risk of miscarriage, preeclampsia, placental abruption, and preterm delivery. These risks apply to both overt and subclinical forms, though they are more pronounced when hypothyroidism is severe.
Iodine and Prevention
Iodine is the raw material your thyroid uses to manufacture hormone, and pregnancy increases your iodine needs substantially. The recommended daily intake for pregnant women is 220 to 290 micrograms, compared to 150 micrograms for nonpregnant adults. Most prenatal vitamins contain iodine, but not all of them, so it’s worth checking the label. Women in regions with moderate to severe iodine deficiency are at particular risk of developing hypothyroidism during pregnancy simply because their thyroid can’t source enough iodine to meet the increased demand.
Postpartum Thyroiditis
Pregnancy can also trigger hypothyroidism after delivery, not just during it. Postpartum thyroiditis is an autoimmune inflammation of the thyroid that develops within the first year after giving birth. It follows a characteristic pattern: an initial phase of excess thyroid hormone release (typically 1 to 4 months postpartum) as the inflamed gland leaks stored hormone, followed by a hypothyroid phase around 4 to 8 months postpartum as the gland’s reserves are depleted. About 43 percent of women with postpartum thyroiditis experience only the hypothyroid phase without the initial overactive period.
Symptoms during the hypothyroid phase include fatigue, weight gain, cold sensitivity, and depression. These overlap heavily with the normal challenges of new parenthood, which is why postpartum thyroiditis often goes undiagnosed. Most cases resolve within 12 months, but the recovery rate isn’t universal. Between 20 and 50 percent of women remain hypothyroid at the one-year mark, and up to 20 percent, particularly those with TPO antibodies, develop permanent hypothyroidism requiring lifelong treatment.
What Treatment Looks Like
Treatment for hypothyroidism during pregnancy is straightforward: synthetic thyroid hormone taken as a daily pill. The goal is to keep TSH within the trimester-specific normal range, which requires monitoring through blood tests every 4 to 6 weeks. Women already on thyroid medication before pregnancy are typically advised to increase their dose as soon as pregnancy is confirmed, then fine-tune based on lab results. For women newly diagnosed during pregnancy, treatment usually starts promptly because of the time-sensitive nature of fetal brain development in early pregnancy.
For postpartum thyroiditis, treatment depends on severity. Mild cases may simply be monitored with periodic blood tests while the condition resolves on its own. More symptomatic cases are treated with thyroid hormone replacement, which can be gradually tapered as the thyroid recovers. Women who develop postpartum thyroiditis after one pregnancy have a higher chance of it recurring with subsequent pregnancies.