Hypothyroidism during pregnancy

By trimester

Weeks 1–13

1st trimester

Thyroid hormone needs can rise early, so someone already taking levothyroxine should contact the prescribing clinician promptly after a positive pregnancy test. TSH and free T4 interpretation should use pregnancy and trimester context, with closer checks during the first half of pregnancy.

Weeks 14–27

2nd trimester

Monitoring generally continues every four to six weeks through the first half of pregnancy. Dose changes are based on laboratory results and the clinical picture, not symptoms alone, because tiredness, constipation, and weight change overlap with ordinary pregnancy experiences.

Weeks 28–birth

3rd trimester

NIDDK describes at least one additional thyroid check after 30 weeks for patients under active thyroid care. Keep levothyroxine timing consistent, separate it from iron or calcium, and make a postpartum testing and dose plan before birth.

What does the evidence show for hypothyroidism?

• NIDDK explains that hypothyroidism can be difficult to recognize from symptoms alone in pregnancy because tiredness, weight changes, and other features overlap. Blood tests for TSH and thyroid hormone are used, and results should be interpreted in pregnancy context. 1
• The 2026 ATA guideline distinguishes elevated TSH with low free T4 from elevated TSH with normal free T4. A very high TSH with normal free T4 is termed profound subclinical disease, so the hormone pattern matters as well as the number. 2
• ATA’s 2026 guidance supports levothyroxine for profound subclinical disease. For mild subclinical disease, it allows consideration in the first trimester but advises against starting it after that trimester, with follow-up testing instead. Timing boundaries require individual interpretation. 2
• Levothyroxine is taken consistently on an empty stomach, commonly 30 to 60 minutes before breakfast. Calcium carbonate and ferrous sulfate should be separated by at least four hours because they can interfere with absorption, which is especially relevant to prenatal vitamins containing iron or calcium. 3
• A secondary analysis of two trial cohorts found that participants with gestational subclinical hypothyroidism had a higher frequency of later overt hypothyroidism or thyroid medicine use than those with isolated low free T4, particularly with higher TSH or thyroid peroxidase antibodies. Follow-up findings do not establish the benefit of medicine during pregnancy. 4

When should I call my provider about hypothyroidism?

Contact the prescribing clinician promptly after a positive pregnancy test if you take levothyroxine, or if thyroid testing is markedly abnormal. Seek urgent care for confusion, fainting, severe weakness, trouble breathing, chest pain, an unusually slow heart rate with feeling very ill, or rapidly worsening swelling, because these are not routine pregnancy symptoms. Trust your read on your own body. If something feels off beyond the list above, please consult your healthcare provider rather than waiting it out.