hypothyroidism and pregnancy

Last edited 04/2021 and last reviewed 12/2021

Both overt and subclinical hypothyroidism is associated with significant risk to both the mother and the developing foetus (1). The prevalence of

• overt hypothyroidism during pregnancy is estimated to be 0.3–0.5%
• subclinical hypothyroidism during pregnancy is estimated to be around 2–3%

The main causes of hypothyroidism during pregnancy are:

• iodine deficiency - affects more than 1.2 billion people
• chronic autoimmune thyroiditis - when iodine intake is adequate (2)

During pregnancy thyroxine requirements may increase by up to 50%; thyroid function tests are undertaken each trimester. During the first trimester, higher circulating human chorionic gonadotrophin (hCG) causes lowering of TSH levels, and more reliance should be placed on maintaining T4 at the upper end of the normal range (3).

Management of women with hypothyroidism during pregnancy requires specialist supervision.

In the postpartum period thyroxine requirements return to the pre-pregnancy levels.

Siskind et al note (5):

In consideration of hypothyroidism in patients who are pregnant, planning pregnancy, affected by infertility, or postpartum

• untreated hypothyroidism during pregnancy
  • may adversely affect maternal and fetal outcomes
    
    
• all women seeking care for infertility warrant evaluation for hypothyroidism
  • if overt hypothyroidism, treatment may restore normal fertility
    
    
• for women planning conception but without history of infertility,thyroid disease, or known thyroid antibody positivity, recommend testing patients with signs, symptoms, or risk factors for hypothyroidism, but do not recommend routine screening (5)
  
  
• postnatally, women may develop postpartum thyroiditis (PPT)
  • can include a thyrotoxic phase followed by a hypothyroid phase
  • maintain a high level of suspicion for thyroid dysfunction in patientsin the postpartum period, and check TSH level if symptoms (including depression) arise
    
    
• women with a history of PPT
  • should have a TSH level measured annually, as up to half of patients in whom the hypothyroid phase of PPT initially resolves go on to develop permanent hypothyroidism

Notes:

• during the first trimester of pregnancy maternal thyroid hormone is responsible for normal foetal neurological development (till the foetal thyroid gland becomes active). So there is an increased need of maternal thyroxine in pregnant women (4)
• due to the increased demand of maternal thyroid hormone, the requirement for iodine is also increased during pregnancy. In areas of iodine deficiency this becomes a significant problem and inadequate intake of iodine may lead to hypothyroidism (4)
• in pregnancy, changes in the immune system take place with relapse or de novo development of autoimmune thyroiditis (4). Women with autoimmune thyroiditis who were euthyroid early in pregnancy, are at an increased risk of developing hypothyroidism as the pregnancy progresses (1)

Reference:

• (1) Abalovich M et al. Management of thyroid dysfunction during pregnancy and postpartum: an Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2007;92(8 Suppl):S1-47.
• (2) Glinoer D, Abalovich M. Unresolved questions in managing hypothyroidism during pregnancy. BMJ. 2007;335(7614):300-2. 
• (3) Prescriber (2002), 13 (10), 50-68.
• (4) Association for Clinical Biochemistry (ACB), British Thyroid Association (BTA), British Thyroid Foundation (BTF) 2006. UK guidelines for the use of thyroid function tests
• (5) Siskind SM et al. Investigating hypothyroidism. BMJ 2021;373:n993http://dx.doi.org/10.1136/bmj.n993