••• Chapter 3^ Physiology of Normal Pregnancy^37
❍ What percentage of pregnant women will have “hyperthyroid” levels of TSH during each trimester?
- 13% of gravidas in the first trimester.
- 4.5% in the second trimester.
- 1.2% in the third trimester.
The undetectable TSH levels occur in the absence of thyroid disease due to the effects of b-hCG.
❍ Why does rising hCG cause a decrease in the TSH level?
Both contain a homologous alpha-subunit. The hCG may act as TSH and stimulate the pituitary to secrete thyroid
hormones that in turn suppress the release of TSH.
❍ Does the basal metabolic rate increase or decrease in pregnancy and by how much?
Increases. Oxygen consumption increases by 25% as a result of fetal metabolic activity.
❍ At what gestational age does thyroid-binding globulin plateau?
The peak increase begins early in the first trimester with a plateau at approximately 500 nmol/L at 20 weeks until
term.
❍ Describe the trend in the free T4 and free T3 levels in pregnancy.
- Both levels decrease from 6 weeks to a nadir and plateau at 20 weeks.
- Both remain within the normal nonpregnant reference range.
- Both correspond to decreasing thyroxine-binding globulin saturation that decreases from 40% to 30%.
❍ List the six most important factors responsible for calcium metabolism in pregnancy.
- Serum calcium levels.
- Magnesium levels.
- Phosphate levels.
- Parathyroid hormone (PTH).
- Calcitonin.
- Vitamin D.
❍ Why is pregnancy termed a “hyperparathyroid state”?
The feto-maternal unit has the primary goal of transporting calcium across the placenta (by active transport) for
fetal skeletal development. This consumes most of the maternal calcium. Calcium concentration is maintained
within normal range despite the increased and expanding extracellular volume. As calcium needs are very great,
PTH levels are increased by 30% to 50% to bring calcium from the maternal bone, kidney, and intestine into
the serum.
❍ Describe the trend of calcitonin in pregnancy.
Calcitonin is secreted from the parafollicular cells of the thyroid gland. Calcitonin levels have been shown to
increase from 13 to 16 weeks with a peak at 25 weeks (230 pg/mL) then a return to prepregnancy levels at about
35 weeks (200 pg/mL). Not only does the level increase but the responsiveness to hyopcalcemia also increases in
order to protect the maternal skeleton from calcium loss.