Maternal Hyperthyroidism

Hyperthyroidism occurs in 1 to 2 out of every 1,000 pregnancies ^{(Inoue 2009)} with Graves' disease being the most frequent cause (85%: ^{Marx 2008}). It often improves in the third trimester of pregnancy allowing reduction or withdrawal of medication and usually worsens after delivery, requiring reinstatement or reinforcement of treatment, which is safe for the infant whether it is continued from gestation or established during lactation and does not require monitoring of the infant's thyroid function, it is sufficient to control the adequate physical and psychomotor development. ^{(Alexander 2017, Inoue 2009, Marx 2008, Mandel 2001)}

There is general consensus among authors and specialized societies to give treatment without interrupting lactation ^{(Ashkar 2023, Dumitrascu 2021, Romeo 2020, Cuff 2019, Alexander 2017, Poppe 2012, Lazarus 2012, Mestman 2012)}. It is convenient to administer the medication immediately after a breastfeeding.  ^{(Ashkar 2023, Hudzik 2016)}

• Propylthiouracil (PTU) at doses of even 750 mg daily does not alter thyroid function or normal infant development ^{(Inoue 2009, Glatstein 2009, Marx 2008, Azizi 2006, Bartalena 2005, Momotani 2000 and 1989, Cooper 1987, McDougall 1986, Kampmann 1980)}. Other authors limit the safe dose to 300 - 450 mg/day. ^{(Ashkar 2023, Dumitrascu 2021, Alexander 2017, Stagnaro 2011, Mandel 2001)}
• Methimazole (MMI) at doses up to 20 - 30 mg daily does not alter infant thyroid function or psychomotor development. ^{(Ashkar 2023, Dumitrascu 2021, Alexander 2017, Serrano 2014, Stagnaro 2011, Inoue 2009, Glatstein 2009, Marx 2008, Azizi 2006, 2003, 2002, 2000 and 1996, Bartalena 2005, Cooper 1987)}
• Given the theoretical risk of liver toxicity of propylthiouracil, methimazole is considered treatment of choice for hyperthyroidism, especially during lactation, ^{(Hudzik 2016, Serrano 2014, Karras 2012, 2010 and 2009, Azizi 2011)}, although there have been no published cases of infants developing liver disease due to maternal use of PTU or MMI. ^{(Romeo 2020)}
• Carbimazole 50 mg weekly also does not alter thyroid function or normal infant development. ^{(Bartalena 2005, Verd 1998)}
• The radioactivity of Iodine 131 concentrates in breast milk and its long half-life makes it necessary to interrupt lactation in both diagnostic and treatment doses ^{(Alexander 2017, Prunty 2016, Serrano 2014, Am.Thyr.Ass 2011, Stagnaro 2011, Abalovich 2007, Azizi 2006, Dydek 1988)} , being able to measure weekly radioactivity in milk samples in order to resume it ^{(Saenz 2000)}. If required, for diagnostic or control purposes, Iodine 123 with a half-life of 13 hours can be used, suspending lactation for 4 days or Technetium 99m which only requires a 1-day interruption.
• Beta-blockers are necessary to control the thyrotoxicosis state; propranolol or metoprolol at the minimum sufficient dose are compatible with lactation. ^{(Alexander 2017)}

There are no data to support that hyperthyroidism can affect milk production or lactation in general. ^{(Alexander 2017)}

Postpartum thyroiditis, with its clinical phases of hyper- and hypothyroidism, is an autoimmune-caused inflammation that occurs in the first year after delivery, with an incidence of 3 to 16% ^{(Serrano 2014, Muller 2001)}. There are no data to recommend universal screening but there is in women with type 1 diabetes mellitus or associated depression ^{(Abalovich 2007)}. During the hypothyroid phase of postpartum thyroiditis, levothyroxine may need to be added, especially if symptoms are present, they are breastfeeding or trying to become pregnant. ^{(Cuff 2019)}

Hyperthyrodism is associated with increased risk of breast cancer, especially if associated with overweight and/or duration of breastfeeding less than 6 months.^{(Yang 2020)}

There is controversy about its association with hyperprolactinemia. ^{(Sanjari 2016, Onal 2014)}

Anti-TSH receptor antibodies (TRAb) can be found in milk from mothers treated for thyrotoxicosis in the first two months, and may cause transient thyroid disease in the infant ^{(Törnhage 2006)}, usually hyperthyroidism that may require treatment. ^{(Azizi 2011)}

During pregnancy and lactation iodine requirements are increased, being about 250 micrograms (μ) daily and not exceeding 500 μ daily ^{(Southern 2024, Alexander 2017, Serrano 2014, Stagnaro 2011, Abalovich 2007)}. In areas with severe iodine deficiency and no use of iodized salt, postpartum administration of a single oral dose of 400 mg of iodized oil ensures an adequate amount of iodine in breast milk for 6 months. ^{(Bouhouch 2014)}

See below the information of these related products:

• Carbimazole (Safe product and/or breastfeeding is the best option.)
• Iodine (nutritional supplement; micrograms) (Safe product and/or breastfeeding is the best option.)
• Iodine 123 (Unsafe. Moderate/severe adverse effects. Compatible under certain circumstances. Follow-up recommended. Use safer alternative or discontinue breastfeeding from 5 to 7 T ½ . Read Commentary.)
• Iodine-131 (Unsafe. Moderate/severe adverse effects. Compatible under certain circumstances. Follow-up recommended. Use safer alternative or discontinue breastfeeding from 5 to 7 T ½ . Read Commentary.)
• Methimazole (Safe product and/or breastfeeding is the best option.)
• Metoprolol (Safe product and/or breastfeeding is the best option.)
• Propranolol (Safe product and/or breastfeeding is the best option.)
• Propylthiouracil (PTU) (Safe product and/or breastfeeding is the best option.)