II. Physiology: Pregnancy specific Thyroid changes
-
Thyroid Hormone requirements increase by 20-40% in pregnancy (as early as 4 weeks gestation)
- Thyroid Binding Globulin increases (Estrogen mediated)
- Thyroid Hormone (Thyroxine) has a greater volume of distribution
- Placental metabolism of Thyroxine
- HCG is a weak TSH receptor stimulator in early pregnancy (decreases TSH levels), but normalizes by second trimester
- Thyroid function markers are less reliable in pregnancy
III. Indications: Thyroid testing in pregnancy
- Many Prenatal Lab testing protocols routinely include Thyroid Stimulating Hormone (TSH)
- Current or prior Thyroid disorder management
- Hypothyroidism
- Hyperthyroidism
- Thyroid Goiter
- Postpartum Thyroid dysfunction history
- TPO Antibody Positive
- Family History of autoimmune Thyroid disease (e.g. Grave's Disease, Hashimoto's Thyroiditis)
- Type I Diabetes Mellitus
- Autoimmune disorder history
- High dose neck radiation exposure history
- Neontal Hypothyroidism with prior delivery
IV. Labs: Thyroid Testing
- Serum Thyroid Stimulating Hormone (TSH) normal ranges
- Non-pregnanct: 0.3 to 4.3 mIU/ml
- First trimester: 0.1 to 2.5 mIU/ml
- Second trimester: 0.2 to 3.0 mIU/ml
- Third timester: 0.3 to 3.0 mIU/ml
-
Free Thyroxine (Free T4) normal ranges
- Non-pregnant: 0.8 to 1.7 ng/dl
- First trimester: 0.8 to 1.2 ng/dl
- Second trimester: 0.6 to 1.0 ng/dl
- Third timester: 0.5 to 0.8 ng/dl
- References