Key takeaways: Both an underactive and an overactive thyroid can affect how you feel, and an underactive thyroid has been linked to lower milk supply. Postpartum thyroiditis is common in the first year. Thyroid hormone replacement (levothyroxine) is compatible with breastfeeding, and many antithyroid medicines can be used with monitoring. Ask for testing if symptoms fit.
Your thyroid helps regulate energy, metabolism, and many body functions. Thyroid problems are common around pregnancy and can affect breastfeeding.
Hypothyroidism (Underactive Thyroid)
Symptoms: tiredness beyond normal new-parent fatigue, feeling cold, constipation, dry skin, hair loss, low mood, weight gain.
Milk supply: untreated hypothyroidism has been linked to low milk supply in clinical reports, though research is limited. Treating it helps overall health.
Treatment: levothyroxine replaces thyroid hormone. It’s considered compatible with breastfeeding. Your dose may need adjusting after birth.
Hyperthyroidism (Overactive Thyroid)
Symptoms: racing heart, anxiety, shakiness, sweating, trouble sleeping, weight loss, feeling hot.
Milk supply: some people report oversupply or letdown changes, but research is limited.
Treatment: antithyroid medicines such as methimazole and propylthiouracil can be used while breastfeeding at moderate doses, according to American Thyroid Association guidance, with monitoring. Radioactive iodine treatment requires stopping breastfeeding; discuss timing carefully with your doctor.
Postpartum Thyroiditis
This is inflammation of the thyroid in the first year after birth. It often has two phases:
- A temporary overactive phase (often 1 to 4 months after birth).
- A temporary underactive phase (often 4 to 8 months after birth).
Some people only have one phase. Many recover, but some develop lasting hypothyroidism. It’s more common in people with thyroid antibodies or type 1 diabetes.
Why It’s Easy to Miss
Many symptoms overlap with normal postpartum life, such as tiredness, mood changes, and hair shedding. Trust your instincts if something feels off.
Testing
A simple blood test (TSH, and sometimes free T4 and antibodies) can check thyroid function. Ask your doctor if you have symptoms, a history of thyroid disease, or low supply without a clear cause.
Imaging and Scans
Some thyroid scans use radioactive materials that may require pausing breastfeeding. Ask before any scan.
See your doctor if you have: ongoing exhaustion, racing heart, feeling very hot or cold, mood changes, or low milk supply with no clear cause.
The Bottom Line
Thyroid problems are common after birth and treatable. Most thyroid medications are compatible with breastfeeding. Ask for testing if symptoms fit.
Want more support for your baby’s first year? My book, From First Breath to First Birthday, is available on Amazon.
Sources
- Alexander EK, Pearce EN, Brent GA, et al. 2017 guidelines of the American Thyroid Association for the diagnosis and management of thyroid disease during pregnancy and the postpartum. Thyroid. 2017;27(3):315-389.
- National Library of Medicine. Levothyroxine; Methimazole; Propylthiouracil. Drugs and Lactation Database (LactMed). https://www.ncbi.nlm.nih.gov/books/NBK501922/
- American Thyroid Association. Postpartum thyroiditis. https://www.thyroid.org
- Lawrence RA, Lawrence RM. Breastfeeding: A Guide for the Medical Profession. 9th ed. Elsevier; 2022.
This article is for general education and is not a substitute for individualized medical advice. Always talk with your baby’s health care provider or your own provider about your specific situation. If you think your baby or you may have a medical emergency, call 911.
