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Key takeaways
Synthroid is a drug for hypothyroidism that’s safe for breastfeeding mothers.
Synthroid may help increase the mother’s milk supply.
While a mother may experience side effects from the medicine, the baby will not.
Synthroid (levothyroxine) is a popular prescription medication used for treating hypothyroidism. Those with hypothyroidism don’t produce enough thyroid hormone, so healthcare providers treat patients with a synthetic hormone, like Synthroid.
In most cases, breastfeeding mothers can safely use Synthroid. What’s even better is that taking Synthroid while breastfeeding can benefit both the postpartum mother and the baby.
Can you take Synthroid while breastfeeding?
“Yes, Synthroid (levothyroxine) can be taken while breastfeeding,” explains Jessica Madden, MD, medical director at Aeroflow Breastpumps and a board-certified pediatrician, neonatologist, and lactation consultant. “It is classified as an L1 medication during lactation, which means that it is not associated with a risk for adverse side effects in infants whose mothers take it while breastfeeding.”
Before we get further into Synthroid and breastfeeding, let’s break down the basics of Synthroid and why it is used. Levothyroxine, sold under the brand name Synthroid, is the human-made, or synthetic version, of the natural thyroid hormone thyroxine. Several thyroid disorders can cause an underactive thyroid.
The types of hypothyroidism are broken down into three categories:
- Primary hypothyroidism: The primary form of hypothyroidism is caused by an issue in the thyroid gland. An autoimmune disease like Hashimoto’s thyroiditis may be the underlying cause of this dysfunction. Latrogenic hypothyroidism, where a person produces abnormally low levels of the thyroid, is another primary type. This form may happen if someone has thyroid cancer and undergoes a thyroidectomy (where part or all of the thyroid is removed). They will need additional thyroid hormones throughout their lives.
- Secondary hypothyroidism: This form of hypothyroidism is caused by issues in the pituitary gland. Problems with the pituitary gland can cause an insufficient release of the thyroid-stimulating hormone (TSH), which determines how much thyroid hormone is produced in the thyroid gland.
- Tertiary hypothyroidism: The third type is the rarest form of hypothyroidism and is related to the hypothalamus. Problems in the hypothalamus may result in insufficient thyrotropin-releasing hormone (TRH). TRH regulates the thyroid gland and endocrine system.
All types of hypothyroidism come with many noticeable symptoms, which may become more severe over time. To combat these symptoms, patients can take a thyroid medication, like Synthroid. Common symptoms of hypothyroidism include:
- Fatigue
- Constipation
- Dry skin
- Weight gain
- Muscle weakness
- Muscle aches
- Slowed heart rate
- Depression
- Memory problems
It’s also possible to have an overactive thyroid disorder, like Graves’ disease. This diagnosis would require antithyroid drugs, which work to decrease levels of thyroid hormones (the opposite of Synthroid). If you suspect you or someone you know may have thyroid problems, speak to a healthcare provider about medical advice. A specialist, like an endocrinologist, can do thyroid function tests, which consist of blood tests, imaging, and antibody tests.
Now, let’s get back to taking Synthroid and breastfeeding babies. Along with all the symptoms of hypothyroidism mentioned above, there are also symptoms specific to new mothers. “Hypothyroidism can interfere with milk production, so it’s especially important for mothers with hypothyroidism to take Synthroid during lactation,” said Dr. Madden. If hypothyroidism is causing a low breast milk supply, it’s possible that Synthroid can increase the body’s milk production.
How long should I wait to breastfeed after taking Synthroid?
“It’s generally safe to breastfeed without waiting after taking Synthroid, but it’s still a good idea to ask your doctor first if you’re worried,” says Raj Dasgupta, MD, internist and program director at Huntington Health Hospital in California.
“Synthroid (levothyroxine) has a half-life of 5-7 days, but only a tiny amount of that goes into your breast milk,” Dr. Dasgupta explains. A half-life of 5 to 7 days means the amount of the drug in the body is reduced by half within a week. However, even immediately after taking a dose of Synthroid, the amount of the drug that goes into the human milk supply is minimal. Of the tiny amount that makes it to the breast milk, research has found that taking “levothyroxine during breastfeeding indicates no adverse effects in infants.”
“There are not any case reports of side effects in babies whose mothers take Synthroid while breastfeeding,” Dr. Madden agrees.
The American Thyroid Association even recommends “that subclinical and overt hypothyroidism should be treated with levothyroxine in lactating women seeking to breastfeed.” The association further added that adequate treatment with levothyroxine may normalize milk production in mothers with a low milk supply.
Side effects of Synthroid while breastfeeding
“If a mother takes too high of a dose, she may develop symptoms of hyperthyroidism,” says Dr. Madden. Hyperthyroidism happens when the body makes too much of the thyroid hormone.
Symptoms of hyperthyroidism from taking high doses of Synthroid that need to be adjusted may include:
- Sweating
- Rapid heart rate
- Weight loss
- Insomnia
While the mother may experience these adverse events while on too high of a dose of Synthroid, Dr. Madden noted, “her baby is not at risk of these side effects.”
Other common side effects for Synthroid users (typically found in those who have overtreatment) can include:
- Hair loss
- Nervousness or irritability
- Chest pain
- Shortness of breath
- Changes in appetite
- Changes in menstrual periods
- Muscle weakness
- Tremors or muscle spasms
- Intolerance to heat
- Decreased bone density
- Impaired fertility
How to take Synthroid while breastfeeding
You should take Synthroid while breastfeeding the same way you would take it if you weren’t breastfeeding. It will be important to monitor your weight post-birth for any fluctuations, as Synthroid’s dose is based on body weight. Otherwise, you’ll take the daily tablet dose as normal while breastfeeding.
The bottom line
Synthroid is safe to take while breastfeeding for the mother and baby. It can even help breastfeeding mothers increase their milk supply. While an almost undetectable amount of Synthroid will go into the milk, the side effects of the drug will not be passed to the baby.
Sources
- Thyroid, Drugs and Lactation Database (LactMed) (2021)
- Levothyroxine, Stat Pearls (2024)
- The quantity of thyroid hormone in human milk is too low to influence plasma thyroid hormone levels in the very preterm infant, Clinical Endocrinology (2002)