Sub-section: Thyroid Section: Endocrine
Physiologic Changes
- Increased metabolic needs
- hCG secreted by Placenta
- Significant homology with TSH ⇒ Active at TSH-R
- Elevated levels of serum TBG
- Requires greater levels of total T4 and T3 to maintain adequate levels of free hormone
- Overall, anticipated normal ranges of TSH concentration are lower during pregnancy with the lowest levels seen during the first trimester
Hypothyroidism
- Prevalence in pregnancy: 2% to 3%
- Foetus depends entirely on maternal thyroid hormone until 10/40
- When foetal thyroid begins producing small amounts of thyroid hormone
- Maternal hypothyroidism is associated with a wide range of poor pregnancy outcomes
- Spontaneous abortion, foetal death
- Preterm delivery
- Pregnancy-induced HTN
- Gestational diabetes
- Anaemia
- Post-partum haemorrhage
- Placental abruption and preterm labour
- Pre-eclampsia
- Caesarean section
- Early embryo loss
- Subclinical hypothyroidism has been associated with 3 x risk of placental abruption and an almost 2 x risk of preterm labour
- Measure TFTs within 30 to 40 days of the first positive pregnancy test, then every 4-6/52
- Treat pregnant women with subclinical or overt hypothyroidism to maintain serum TSH
Hyperthyroidism
- Estimated prevalence: 0.4% to 1.7%
- Most common cause of biochemical hyperthyroidism: hCG mediated
- Hyperthyroidism usually mild and resolves spontaneously
- Treatment with antithyroid drugs is not recommended
- Grave’s Disease accounts for 85% to 90% of overt hyperthyroidism in pregnant women
- Untreated overt maternal hyperthyroidism associated with the risk of low birth weight, severe preeclampsia, miscarriages, maternal congestive heart failure, stillbirth, and foetal growth restriction
- RAI is contraindicated in pregnancy (at diagnostic and therapeutic doses)
- For medical treatment of hyperthyroidism in pregnancy
- PTU is recommended in the first trimester (possible hepatotoxicity)
- Methimazole is recommended in the second and third trimesters (Contraindicated 1st trimester - teratogenicity)
- Thyroidectomy occasionally indicated
- May be performed if rapid control of hyperthyroidism is needed and antithyroid medications cannot be used because of allergies or non-compliance
- If thyroidectomy necessary, preferentially performed during the second trimester
- Teratogenicity and foetal loss risk in 1st Trimester
- Risk of preterm labour in the 3rd Trimester
- Pre-op rx may include 10 to 14 days of iodine, antithyroid medications, and beta blockers