An excerpt from Marilyn M. Shannon’s book, Fertility, Cycles & Nutrition.
At some point after giving birth, often before the baby is six months old, a small percentage of women experience thyroid problems. About one to four months after the birth, some new mothers (approximately 4 percent) develop an overactive thyroid gland. They may feel heart palpitations and fatigue, both of which develop rather rapidly. They may develop a small, painless goiter. About two-thirds of such women will return to normal on their own, or they may actually go on to develop low thyroid function.
Approximately two to five percent of women develop low thyroid function (hypothyroidism) about four to eight months postpartum, but sometimes even earlier. Often it corrects itself on its own by the time the baby is a year old, but replacement of thyroid hormone may be necessary, at least temporarily. Of the women who develop either overactive thyroid or hypothyroidism after giving birth, about one-third develop permanent hypothyroidism, which must be treated medically.
The NFP chart
I have observed very low temperatures, often in the range of 96.8°F and below, on the charts of some women with young babies, reflecting the low thyroid function that may occur postpartum. I have also seen the temperatures rise over a period of two weeks to a month after such women have supplemented vitamins, minerals, and essential fatty acids as recommended [elsewhere in this book]. If you see very low temperatures on your chart, whether or not you are breastfeeding and whether or not you have had your periods return, give the [recommended supplements] a try for a month or two. However, you should be in touch with a medical doctor as well.
Mary Shomon, author of The Thyroid Hormone Breakthrough, explains postpartum thyroid problems thoroughly in an excellent chapter dedicated to this topic. … For example, she explains how women with previous thyroid problems may have to be tested regularly after they have a baby because their needs for thyroid medication may change. She explains how either hyper- or hypothyroidism can reduce milk production in a nursing mother, and offers strategies to increase milk output.
Mary Shomon also makes the distinction between the hair loss that is common after a woman has a baby versus hair loss that is a symptom of low thyroid function. In addition to comprehensive multivitamin/multimineral supplements, she recommends 500 mg of evening primrose oil daily to overcome hair loss. However, if you are nursing your baby, instead of evening primrose oil, I recommend flax oil (five to ten one-gram capsules, or one to two teaspoons daily), along with vitamin E (400 IU).
— Marilyn M. Shannon is a tenured, part-time instructor of biology at Indiana University Purdue University at Fort Wayne, Indiana, where she has taught human anatomy and physiology for more than 30 years. Her book, Fertility, Cycles & Nutrition is a staple for many NFP families and is available here.
 Casey, B., and Leveno, K. “Thyroid disease in pregnancy” (Obstet. Gynecol, 2006, 108:1283-1292).
 Casey, B., and Leveno, K. (2006).
 Shomon, M. The Thyroid Hormone Breakthrough (New York: Collins, 2006, p. 231).