Short answer: The thyroid gland controls metabolism and also affects reproduction. Untreated hypothyroidism (underactive thyroid) appears to make it more difficult to conceive and increases the risk of miscarriage, whereas a well-treated thyroid usually does not pose an obstacle. The research situation is partly uncertain, especially regarding mild disorders. If you wish to have children, it may be reasonable to check your thyroid levels.
What does the thyroid gland do – and why is it important for fertility?
The thyroid gland produces hormones (primarily T4 and T3) that regulate the body's metabolism. These hormones interact with the hormones that control ovulation and the menstrual cycle, which is one of the explanations why thyroid function can be important for fertility. In hypothyroidism (underactive thyroid), too little hormone is produced, often resulting in elevated TSH. The evidence suggests that both overactive and underactive thyroid function can disrupt the cycle and thus the ability to conceive.
Hypothyroidism and the ability to conceive
According to 1177 Vårdguiden, it is usually not more difficult to conceive if you have hypothyroidism that is treated with thyroid hormone, whereas untreated hypothyroidism can make it more difficult. Disturbances in thyroid function have been linked in studies to irregular ovulation and an increased risk of miscarriage. Therefore, a well-treated thyroid, rather than the diagnosis itself, is usually central before a pregnancy.
Subclinical hypothyroidism and thyroid antibodies
In subclinical (mild) hypothyroidism, TSH is slightly elevated while T4 is still within the reference range. The research situation here is more uncertain. Reviews suggest that the presence of thyroid antibodies (a sign of autoimmune thyroid disease) may be linked to a slightly higher risk of unexplained reduced fertility and miscarriage, but the correlations are not entirely unequivocal and causality is difficult to establish. A guideline from the American Society for Reproductive Medicine (ASRM, 2024) summarizes that the evidence for the benefit of treatment for mild disorders is limited and partly contradictory.
Increased need during pregnancy
During pregnancy, the body's need for thyroid hormone increases, as the fetus is dependent on the mother's hormones during the initial period. Those who are already being treated for hypothyroidism therefore often need to adjust their dose early in pregnancy, in consultation with healthcare. This is one of the reasons why thyroid levels are often checked when trying to conceive and in early pregnancy.
What can you do if you wish to have children?
If you have a known thyroid condition, symptoms such as fatigue, cold sensitivity or weight changes, or if you have had difficulty conceiving or experienced miscarriages, it may be reasonable to discuss your thyroid with your healthcare provider. A simple blood test (TSH and often T4) provides a picture of the function. Decisions about testing, target values, and potential treatment should always be made individually in consultation with healthcare.
Frequently asked questions
Can hypothyroidism prevent me from getting pregnant?
Untreated hypothyroidism can in some cases make it more difficult to conceive, partly by affecting ovulation. With adequate treatment, the starting point is usually significantly better. This should be assessed individually by healthcare professionals.
Should I check my thyroid before trying to get pregnant?
It may be reasonable, especially with symptoms, known thyroid disease, previous miscarriages, or difficulty conceiving. Your healthcare provider can determine if a TSH test is warranted in your specific situation.
Does thyroid medication affect the fetus?
According to 1177 Vårdguiden, levothyroxine (the same hormone the body normally produces) is not harmful to the fetus, but the dose may need to be adjusted during pregnancy. Always follow your healthcare provider's advice.
This text is general information and does not constitute medical advice. For questions about thyroid, fertility or pregnancy – always consult a doctor or other healthcare professional who is familiar with your situation.