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Hypothyroidism is common in pregnancy with an estimated prevalence of 2-3% and 0.3-0.5% for subclinical and overt hypothyroidism respectively. [8] Endemic iodine deficiency accounts for most hypothyroidism in pregnant women worldwide while chronic autoimmune thyroiditis is the most common cause of hypothyroidism in iodine sufficient parts of the world.
Thyroid hormones, T4 and TSH, diffuse across the placenta traveling from the mother to fetus for 10–12 weeks before the fetus’s own thyroid gland can begin synthesizing its own thyroid hormones. [2] The mother continues to supply some T4 to the fetus even after he/she is able to synthesize his/her own.
Subclinical hypothyroidism is a biochemical diagnosis characterized by an elevated serum TSH level, but with a normal serum free thyroxine level. [48] [49] [50] The incidence of subclinical hypothyroidism is estimated to be 3-15% and a higher incidence is seen in elderly people, females and those with lower iodine levels. [48]
Of those women who experience hypothyroidism associated with postpartum thyroiditis, one in five will develop permanent hypothyroidism requiring lifelong treatment. Postpartum thyroiditis is believed to result from the modifications to the immune system necessary in pregnancy, and histologically is a subacute lymphocytic thyroiditis.
In the US, hypothyroidism and hyperthyroidism were respectively found in 4.6 and 1.3% of the >12y old population (2002). [3] In some types, such as subacute thyroiditis or postpartum thyroiditis, symptoms may go away after a few months and laboratory tests may return to normal. [4] However most types of thyroid disease do not resolve on their own.
[4] [5] Presence of these types of complications can have implications on monitoring lab work, imaging, and medical management during pregnancy. [4] Severe complications of pregnancy, childbirth, and the puerperium are present in 1.6% of mothers in the US, [6] and in 1.5% of mothers in Canada. [7]
Subclinical hyperthyroidism in pregnancy is associated with an increased risk of pre-eclampsia, low birth weight, miscarriage and preterm birth. [50] Propylthiouracil is the preferred treatment of hyperthyroidism (both overt and subclinical) in the first trimester of pregnancy as it is associated with less birth defects than methimazole. [50]
Thyroid function tests (TFTs) is a collective term for blood tests used to check the function of the thyroid. [1] TFTs may be requested if a patient is thought to suffer from hyperthyroidism (overactive thyroid) or hypothyroidism (underactive thyroid), or to monitor the effectiveness of either thyroid-suppression or hormone replacement therapy.