Lynn,

It is possible to breastfeed when hyperthyroid. See Lawrence p 573-4, and then the four annotated citations below.  

You can also ask for the considered medications and check them in Hale. When push comes to shove, I do my homework first and then I take it to the *pediatrician* and pitch my case. The ped is the one in charge of infant health, and if they ok it, the prescribing doctor is relieved of the burden.

~Lisa Marasco, MA, IBCLC, FILCA


Glatstein, M. M., Garcia-Bournissen, F., Giglio, N., Finkelstein, Y., & Koren, G. (2009). Pharmacologic treatment of hyperthyroidism during lactation. Can Fam Physician, 55(8), 797-798. doi: 55/8/797 [pii]  
Abstract:
 QUESTION: I have a patient who has hyperthyroidism due to Graves disease. She was taking methimazole but discontinued when she found out she was pregnant. She is currently close to delivery and might require antithyroid therapy in the postpartum period. Can methimazole cross into human milk, and is breastfeeding safe for her infant? ANSWER: The exposure of infants to methimazole or propylthiouracil through breast milk is minimal and not clinically significant. Women with hyperthyroidism using methimazole or propylthiouracil should not be discouraged from breastfeeding, as the benefits of breastfeeding largely outweigh the theoretical minimal risks.

Lao, T. (2005). Management of Hyperthyroidism and Goitre in Pregnancy, and Postpartum Thyroiditis. JOURNAL OF PAEDIATRICS OBSTETRICS AND GYNAECOLOGY, 31(4), 155. 
Abstract: 
Thyroid disorders are an important cause of adverse pregnancy outcomes. In pregnant women, the incidence of clinical thyroid disorders is 1% to 2%.1 As most of the affected women have the disorders before pregnancy, thyroid disorders constitute the commonest group of pregestational endocrine disorders encountered in pregnancy, more common than pregestational diabetes mellitus. The management protocols of hyperthyroidism, which is usually due to Graves' disease (GD), are now well established, and the safety of breastfeeding in women maintained on antithyroid treatment after delivery has been documented. Goitres may be commonly encountered in pregnancy, but in case of doubt, assessment of thyroid function and ultrasound scanning of the thyroid gland are indicated. Postpartum thyroiditis is an often overlooked problem that may have serious implications for the affected mothers and their families, due to its close association with psychological disturbance and depression. A thorough understanding of the effects of thyroid dysfunction on pregnancy and its appropriate management would optimize not only pregnancy outcome but also the long-term health of the affected women. In this review, the diagnosis and management of hyperthyroidism and goitre in pregnancy, as well as postpartum thyroiditis, which can be considered a continuation of the thyroid disorder with manifestation after delivery, are discussed.


Karras S, Tzotzas T, Kaltsas T, Krassas GE. Pharmacological treatment of hyperthyroidism during lactation: review of the literature and novel data. Pediatr Endocrinol Rev. 2010 Sep;8(1):25-33.
Abstract
Antithyroid drugs (ATD) are used as a first line treatment in thyrotoxicosis. Propylthiouracil (PTU), carbimazole (CMZ) and methimazole (MMI) are available. During absorption CMZ is bioactivated to MMI. Initially, mothers were not allowed to breastfeed during treatment with ATD. Newer studies minimized the risk for mother and infant. PTU should be preferred over MMI due to its lower milk concentration. Recent studies have shown severe hepatic dysfunction for both ATD, but especially for PTU, in hyperthyroid patients. Most of those cases were idiosyncratic, not-dose related and presented a latent period of occurrence. No biomarkers could predict hepatic damage. The American Thyroid Association (ATA) has recommended that PTU should not be prescribed as the first line agent in children and adolescents. Its use might be accepted in the first trimester of pregnancy for severe thyrotoxicosis or for patients with previous MMI adverse reactions. Considering the potential harmful effects of PTU, MMI should be used instead during lactation.

Abalovich, M., Amino, N., Barbour, L. A., Cobin, R. H., De Groot, L. J., Glinoer, D., . . . Stagnaro-Green, A. (2007). Management of Thyroid Dysfunction during Pregnancy and Postpartum: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab, 92(8_suppl), s1-47. doi: 10.1210/jc.2007-0141
Abstract:
Conclusions: Management of thyroid diseases during pregnancy requires special considerations because pregnancy induces major changes in thyroid function, and maternal thyroid disease can have adverse effects on the pregnancy and the fetus. Care requires coordination among several healthcare professionals. Avoiding maternal (and fetal) hypothyroidism is of major importance because of potential damage to fetal neural development, an increased incidence of miscarriage, and preterm delivery. Maternal hyperthyroidism and its treatment may be accompanied by coincident problems in fetal thyroid function. Autoimmune thyroid disease is associated with both increased rates of miscarriage, for which the appropriate medical response is uncertain at this time, and postpartum thyroiditis. Fine-needle aspiration cytology should be performed for dominant thyroid nodules discovered in pregnancy. Radioactive isotopes must be avoided during pregnancy and lactation. Universal screening of pregnant women for thyroid disease is not yet supported by adequate studies, but case finding targeted to specific groups of patients who are at increased risk is strongly supported.

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