Thyroid nodules are common in young women and medical care provided during pregnancy may present the first opportunity for a thyroid nodule to be clinically recognized [390, 391]. A few studies have evaluated the incidence of thyroid nodules and nodule changes associated with pregnancy, in areas with borderline iodine sufficiency or mild iodine deficiency. In a prospective study we have performed in Brussels, thyroid nodules were diagnosed by ultrasound at initial presentation during early gestation in 3% of a cohort of normal pregnancies [169]. Repeat ultrasound performed within a week after delivery revealed a 60% increase in the size of the nodules and the detection of new nodules in 20% of the women. In another study performed in Northern Germany, the presence of thyroid nodules was shown to be significantly greater in women with prior pregnancies compared to nulliparous women (25% versus 9%). Furthermore, women with three or more pregnancies had a higher percentage of thyroid nodules, compared with women who had one or two prior pregnancies (34% versus 21%) [393]. Finally in a study from Southern China, women were evaluated in the 3 trimesters of pregnancy and early postpartum [394]. Thyroid nodules were detected in 15% of women in the 1st trimester, and there was a significant increase in their volume, as well as new nodule formation during pregnancy. Thus, in areas with mild iodine insufficiency, pre-existing nodules are prone to increase in size during pregnancy. Furthermore during the course of pregnancy, new nodules will be detected in approximately 15% of women. Unfortunately, data on nodule growth and formation in iodine replete areas are not available.

The diagnostic evaluation of a thyroid nodule discovered during pregnancy should be similar to that of non-pregnant patients, but the ongoing pregnancy raises an additional concern regarding timing of surgical management [395-399]. The diagnosis and decision-making for treatment and overall management of a nodule diagnosed in pregnancy relies primarily on the results of thyroid ultrasound and fine needle aspiration biopsy (FNAB). Despite the fact that a minority of nodules are potentially malignant, the fear of cancer may be accentuated in pregnant women. Therefore, diagnostic investigation using FNAB is recommended in most pregnant women. A number of studies have suggested that a delay in the work-up of a nodule until after delivery causes no change in final prognosis as compared with surgical resection of a malignant lesion in the second trimester [397, 400]. Knowing the diagnosis via FNAB cytology is, however, often helpful to the mother in planning the postpartum period, including decisions regarding breast-feeding and the potential need for adjunctive therapy with radioiodine after surgical removal of a cancer. Furthermore, a delay in surgical treatment of thyroid cancer beyond a one-year period is not recommended because of increased likelihood of cancer complications [396].

Thyroid ultrasonography is useful to characterize the dominant lesion (solid versus cystic), identify other non-palpable nodules within a nodular goiter, monitor nodule growth, etc. Thyroid ultrasound is also a useful adjunct to guiding the FNAB procedure. FNAB is safe and diagnostically reliable and should be routinely performed when any single or dominant thyroid nodule greater than 1 cm has been discovered [392]. In the particular context of a nodule identified during pregnancy, and because of the potential therapeutic implications, it is highly important that FNAB be carried out and analyzed by experienced teams.

The subsequent management depends on results of FNAB cytology. Most nodular lesions are cytologically benign and do not require surgery. If cytology is suspicious or positive for thyroid cancer, treatment decision-making must take into account the gestational age, the apparent tumor stage and the personal inclination of the patient. If FNAB cytology is highly suggestive of papillary, follicular, or medullary carcinoma, surgery is offered in the 2nd trimester but before fetal viability [401]. Surgery for papillary cancer may be postponed until after delivery, because there is no evidence that pregnancy worsens the prognosis of a well-differentiated thyroid cancer or that waiting until postpartum alters the long term prognosis [395, 397, 400, 402]. When cytology is highly suggestive of a follicular neoplasm, the risk of malignancy is 10%-15% and thyroid surgery can be delayed, if preferred, until a short time after delivery. In patients who need to be reassured or when there is significant growth of the nodule before mid-gestation, surgery is a valid option and should be carried out during the second trimester. For follicular neoplasms with Hurthle cell features (i.e. oncocytes), the patient should be encouraged to have surgery during pregnancy, given the more aggressive behavior of Hurthle cell carcinoma [403]. Finally, when a nodule is discovered in the third trimester, further work-up and treatment can be delayed until after delivery [404, 405].

Overall, most experts in the field agree that pregnancy does not worsen the prognosis of differentiated thyroid carcinoma and that there is, therefore, no justification to recommend interrupting the pregnancy. In a recent study, the long-term outcome of a large group of women diagnosed with thyroid cancer was evaluated [406]. The authors observed no significant difference in outcome, compared with an age-matched non pregnant cohort. Furthermore, there was no adverse effect of surgery performed during pregnancy on the outcome of pregnancy.