Breastfeeding and Thyroid Problems: Diagnostic Testing & Treatments

By Kelly Bonyata, IBCLC

Following are various diagnostic tests and treatments that moms with thyroid problems might encounter. The information summarized below is only a general overview. For detailed information, please review the references listed below with your health care provider.

Thyroid screen

This is a blood test that checks thyroid function. Blood tests are compatible with breastfeeding. Tests may include:

  • TSH (thyroid stimulating hormone, produced by the pituitary gland)
  • total T4 (thyroxine, a thyroid hormone) and/or free T4
  • total T3 (triiodothyronine, a thyroid hormone) and/or free T3
  • Thyroid Binding Globulin (TBG)
  • Thyroid Stimulating Antibodies (TSAb)
  • Thyroid Stimulating Immunoglobulin (TSI)

Thyroid medications

  • levothyroxine (Synthroid, Levoxyl) synthetic T4 hormone (1)
  • liothyronine (Cytomel) synthetic T3 hormone
  • combination of T3 & T4 (Thyrolar/Liotrix, Armour Thyroid)
Info on selected thyroid meds
Name of medication
Levothyroxine (T4)
L1 (safest)
Liothyronine (T3)
L2 (safer)
** Per Medications’ and Mothers’ Milk by Thomas Hale, PhD (2014 edition).

(1) “Most studies indicate that minimal levels of maternal thyroid are transferred into human milk, and further, that the amount secreted in extremely low and insufficient to protect a hypothyroid infant even when nursing… it is generally recognized that some thyroxine will transfer but the amount will be extremely low… (Hale 2014, p. 649-50)

(2) “From these studies it is apparent that only exceedingly low levels of T3 are secreted into human milk and are insufficient to protect an infant from hypothyroidism. (Hale 2014, p. 657-58)

Anti-thyroid medications

  • carbimazole (Neo-Mercazole)
  • methimazole (Tapazole)
  • propylthiouracil (PTU)
Info on selected anti-thyroid meds
Name of medication
carbimazole (Neo-Mercazole) L2 (safer) (1)
methimazole (Tapazole) L2 (safer) (2)
propylthiouracil (PTU) L2 (safer) (3)
** Per Medications’ and Mothers’ Milk by Thomas Hale, PhD (2014 edition).

(1) “Carbimazole is a prodrug and is rapidly converted to methimazole.” (Hale 2014, p. 723-24)

(2) Levels of methimazole in milk depend on the maternal dose but appear too low to produce clinical effects.” (Hale 2014, p. 723-24)

(3) “Only small amounts are secreted into breastmilk. Reports thus far suggest that levels absorbed by infant are too low to produce side effects… No changes in infant thyroid have been reported… PTU is the best of antithyroid medications for use in lactating mothers. Monitor infant thyroid function (T4, TSH) carefully during therapy.” (Hale 2014, p. 920)

I’m not hyperthyroid myself, but my sister was diagnosed with Grave’s disease after her first child was born. She breastfed her children while taking anti-thyroid meds for her hyperthyroidism.

Other medications

Some moms with hyperthyroidism are also prescribed beta-blockers (such as Propranolol/Inderal) or calcium channel blockers to relieve the neurological and cardiovascular symptoms of hyperthyroidism. Many of these drugs (including Propranolol/Inderal) are considered to be acceptable for use in breastfeeding mothers.


Ultrasound is compatible with breastfeeding.

Fine needle aspiration (FNA) biopsy

This procedure is compatible with breastfeeding.

You don’t need to stop nursing for a fine needle biopsy. It’s a bit scary to think of (at least it was for me!), but it doesn’t really hurt at all (about like having blood drawn from your arm) and it just takes a few minutes. I’ve had 2 fine needle biopsies and the second was so much easier since I know what to expect.


In this surgery, all or part of the thyroid is removed. Mom can resume breastfeeding as soon after the surgery as she feels up to holding baby.

See also Breastfeeding when mom has surgery.

I had thyroid surgery (a partial thyroidectomy for what turned out to be a benign cold nodule) about a year before my daughter was born, so I was not breastfeeding at the time. As I recall, I was really uncomfortable the day of the surgery, but it got better pretty quickly after that. My neck muscles were sore for weeks – it might take a little creative positioning to help you to breastfeed comfortably. You might want to take a nursing pillow to the hospital to raise baby up so you can see her without moving your head as much, etc. Nursing while lying down might help, too.

I also urge you to nurse right before the surgery and as soon afterwards as you feel you can. Ask your doctor how you can expect to be feeling after surgery. I recall feeling really rotten the first day, but I think I could have nursed just fine as long as someone brought the baby to me. Your milk supply could go down a bit if you miss any nursings, so try to pump if you miss any feedings.

One thing I was worried about with thyroid surgery was the scar… in my case, it was pretty difficult to see after about 6 months (my surgeon said to give it a year), and now (4 years later) I cannot find the scar at all.

Thyroid Scan

This scan can be done using radioactive iodine (I-131 or I-123) or technetium-99m pertechnetate. This test requires temporary weaning for a minimum of 12 hours, depending upon the isotope used (see below). Many times, this test can be skipped and a fine needle aspiration biopsy done instead (which does not require an interruption of breastfeeding).

Technetium-99m has a very short half-life (6.02 hours, compared to 8.1 days for I-131). The amount of time suggested for suspending breastfeeding varies depending on the dosage and form of the isotope (there are many forms of Tc-99m). For some tests, breastfeeding can be resumed immediately; for others it is recommended to suspend breastfeeding for amounts of time varying from 6 hours to 48 hours. See the NRC Table PDF for additional guidance.

I-123 has a half-life of 13.2 hours, and is available in several forms. For one of the forms of the isotope, Hale suggests suspending breastfeeding for 12-24 hours, depending upon the dose, but a couple of other forms/dosages do not require suspension of breastfeeding.

I-131 concentrates in breastmilk and high levels in breastmilk can suppress baby’s thyroid function (or even destroy the thyroid) and increase risk of thyroid cancer. Therefore it is important that breastfeeding be discontinued until breastmilk levels are safe (this depends upon the dose and ranges from 8 days to 106+ days). The half-life for I-131 is 8.1 days. Hale recommends that when I-131 is used, breastmilk samples should be tested with a gamma (radiation) counter before breastfeeding is resumed to ensure that radiation in the milk has returned to safe levels. Lactation Risk Category is L4 (possibly hazardous).

Important note: If you do suspend breastfeeding due to use of radioactive isotopes, it is important to pump regularly during this time. See also Maintaining milk supply when baby is not nursing. You do not need to dump this milk. It can be dated, frozen and used after 5+ half-lives of the radioisotope have passed (after 5 half-lives, 96.9% of the radiation is gone; after 10 half-lives, 99.9% of the radiation is gone). You may also get the milk checked for radiation by your nuclear medicine department.

Reference: Hale, 2002, p. 365-367, 675-676, 689-690. See also Use of Radioisotopes during Lactation.

Radioactive iodine (RAI) Uptake Scan

This scan is done using radioactive iodine (I-131), and is usually done at the same time as a thyroid scan. See the info above on thyroid scans using I-131.

See also Use of Radioisotopes during Lactation.

The type of thyroid scan I had was a radioactive iodine uptake & thyroid scan. I had to drink some radioactive iodine, then come back later in the day to have the scan (like a x-ray). I was not nursing at the time.

For nursing moms, there are usually other diagnostic procedures that can be done instead of a scan, for example, blood tests, ultrasound, and/or fine needle biopsy. The second time I needed diagnostic procedures, my doctor skipped the scan and just did a fine needle biopsy.

Radioactive iodine treatment (RAI)

This treatment is done using radioactive iodine (I-131), but in much larger doses than used for a diagnostic scan. Its purpose is to partially or completely destroy the thyroid. It is recommended that the mother discontinue breastfeeding several days to weeks before this therapy, and pump and dump milk for several weeks after therapy to reduce exposure of the breast tissue to radiation. The US Nuclear Regulatory Commission recommends complete weaning after I-131 is used therapeutically.

See also Use of Radioisotopes during Lactation.

Breastfeeding and Thyroid Problems: Links

Breastfeeding and Thyroid Problems: Studies and References

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