Pregnancy induces major changes in thyroid function1

During pregnancy:

  • The thyroid adapts through changes in thyroid hormone and in the regulation of the hypothalamic-pituitary-thyroid axis
  • Circulating total T4 (TT4) and T4 binding globulin (TBG) concentrations increase by week 7 of gestation, reach a peak by approximately week 16 of gestation and remain high until delivery
  • Serum FT4 measurements are complicated by increased TBG and decreased albumin concentrations, the use of population-based, trimester-specific reference ranges remains the best way to handle this issue
  • Serum TSH concentrations are for the typical patient beginning the late first trimester, week 7-12, upper reference limit of 4.0mU/L, with a gradual return towards the non-pregnant range in the second and third trimesters

Pregnant women at high risk should be screened for overt hypothyroidism

A history of hypothyroidism/hyperthyroidism or current symptoms/signs of thyroid dysfunction

  • Known thyroid antibody positivity or presence of a goiter
  • History of head or neck radiation or prior thyroid surgery
  • Age >30 years
  • Type 1 diabetes or other autoimmune disorders
  • History of pregnancy loss, preterm delivery, or infertility
  • Multiple prior pregnancies (≥2)
  • Family history of autoimmune thyroid disease or thyroid dysfunction
  • Morbid obesity (BMI ≥40 kg/m2)
  • Use of amiodarone or lithium, or recent administration of iodinated radiologic contrast
  • Residing in an area of known moderate to severe iodine insufficiency

Hormone levels of healthy pregnant women differ from those of healthy nonpregnant women1-3

In pregnant women

The pregnancy-specific TSH reference range should be defined as follows:1

When available, population- and trimester-specific reference ranges for serum TSH during pregnancy should be defined by a provider’s institute or laboratory and should represent the typical population for whom care is provided. Reference ranges should be defined in healthy TPOAb-negative pregnant women with optimal iodine intake and without thyroid illness (Strong recommendation, high-quality evidence).

  • When this goal is not feasible, pregnancy-specific TSH reference ranges obtained from similar patient populations and performed using similar TSH assays should be substituted (Strong recommendation, high-quality evidence).
  • If internal or transferable pregnancy-specific TSH reference ranges are not available, an upper reference limit of ~4.0 mU/L may be used. For most assays, this limit represents a reduction in the nonpregnant TSH upper reference limit of ~0.5 mU/L (Strong recommendation, moderate-quality evidence).

Treatment goal

To normalize maternal serum TSH values within the trimester-specific pregnency reference range1

In non-pregnant women

The American Association of Clinical Endocrinologists classify a normal TSH range as:*

  • 0.45–4.12 mIU/L

Adjusting SYNTHROID dosage1-3

Treated hypothyroid patients who are planning pregnancy

  • In hypothyroid women treated with LT4 who are planning pregnancy, serum TSH should be evaluated preconception,and LT4 dose adjusted to achieve a TSH value between the lower reference limit and 2.5 mU/L (Strong recommendation, moderate-quality evidence).

Treated hypothyroid patients who are newly pregnant

  • Hypothyroid patients receiving LT4 treatment with a suspected or confirmed pregnancy (e.g., positive home pregnancy test) should independently increase their dose of LT4 by ~20%–30% and urgently notify their caregiver for prompt testing and further evaluation. One means of accomplishing this is to administer two additional tablets weekly of the patient’s current daily LT4 dosage (Strong recommendation, high-quality evidence).

Following delivery

  • LT4 should be reduced to the patient’s preconception dose. Additional thyroid function testing should be performed at approximately 6 weeks post partum (Strong recommendation, moderate-quality evidence).
  • Some women in whom LT4 is initiated during pregnancy may not require LT4 post partum. Such women are candidates for discontinuing LT4. The decision to discontinue LT4, if desired, should be made by the patient and their caregiver. If LT4 is discontinued, serum TSH should be evaluated in approximately 6 weeks (Weak recommendation, moderate-quality evidence).

References:

  1. 1.Garber JR, Cobin RH, Gharib H, et al. Clinical practice guidelines for hypothyroidism in adults: cosponsored by the American Association of Clinical Endocrinologists and the American Thyroid Association Endocr Pract. 2012;18(6):988-1028. AACE and ATA Taskforce on Hypothyroidism in Adults.
  2. 2.Clinical practice guidelines for hypothyroidism in adults: cosponsored by the American Association of Clinical Endocrinologists and the American Thyroid Association. Thyroid 2012; 22(12):1200-35.