I. Introduction II. The Regulation of Thyroid Function in Normal Pregnancy A. The thyroid hormone transport proteins B. The thyroid hormones 1. Total thyroid hormones 2. Free thyroid hormones 3. Peripheral metabolism of thyroid hormones C. The serum levels of thyroglobulin (TG) D. The metabolism of iodine E. The hypothalamic-pituitary control of thyroid function and the role of hCG 1. Hypothalamic-pituitary-thyroid axis (HPTA) 2. Regulation of serum TSH 3. Thyrotropic action of hCG F. A global view of thyroidal economy in pregnancy III. Pathological Alterations of Thyroidal Regulation Associated with Pregnancy A. IDD 1. Consequences of iodine deficiency during pregnancy 2. Assessment of increased thyroidal stimulation 3. Gestational goitrogenesis and its prevention by iodine supplementation 4. Consequences of iodine deficiency for the offspring B. Hypothyroidism and pregnancy 1. Fertility and pregnancy outcome in hypothyroid women 2. Thyroid hormone replacement in the hypothyroid pregnant woman 3. Subclin...

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The Regulation of Thyroid Function in Pregnancy: Pathways of Endocrine Adaptation from Physiology to Pathology

I. Introduction II. Maternal Physiology and Pathophysiology During Pregnancy A. Maternal adaptive goals during pregnancy B. Mineral ions and calcitropic hormones C. Intestinal absorption of calcium D. Renal handling of calcium E. Skeletal calcium metabolism F. Primary hyperparathyroidism G. Hypoparathyroidism and pseudohypopara- thyroidism H. Summary III. Fetal-Placental Physiology and Pathophysiology A. Fetal adaptive goals B. Mineral ions and calcitropic hormones C. Fetal-placental calcium transport D. Renal handling of calcium and the amniotic fluid E. Skeletal calcium metabolism F. Fetal response to maternal hyper- or hypoparathy-roidism G. Integrated fetal calcium homeostasis H. Summary IV. Maternal Physiology and Pathophysiology During Lactation A. Maternal adaptive goals during lactation B. Mineral ions and calcitropic hormones C. Intestinal absorption of calcium D. Renal handling of calcium E. Skeletal calcium metabolism F. Hypoparathyroidism and pseudohypoparathy- roidism G. Summary V. Neonatal P...

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Maternal-fetal calcium and bone metabolism during pregnancy, puerperium, and lactation.

An ideal battery of tests would include indices of bone resorption and formation. They should be unique to bone, reflect total skeletal activity, and should correlate with traditional measures of bone remodeling activity, such as radiocalcium kinetics, histomorphometry, or changes in bone mass. Factors that confound their measurement, such as circadian rhythms, diet, age, sex, bone mass, liver function, and kidney clearance rates, should be clearly defined (Fig. 9). To date, no bone marker has been established to meet all these criteria, and each marker may have its own specific advantages and limitations. There are still questions that must be answered before there can be complete confidence in the information gained from measurement of any of the bone markers. Furthermore, it should be emphasized that none of the markers are diagnostic for any particular bone disease and cannot be used for this purpose in individual patients. Nevertheless, recent advances in research and development have provided assays with increased specificity, sensitivity, and availability. Because of this, bone markers can be used for a variety of important purposes: as tools for basic bone biology research, for defining general physiological phenomenon in clinical studies or drug trials, and for following individual patients.

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Molecular Basis and Clinical Application of Biological Markers of Bone Turnover

During pregnancy and lactation, female physiology adapts to meet the added nutritional demands of fetuses and neonates. An average full-term fetus contains ∼30 g calcium, 20 g phosphorus, and 0.8 g magnesium. About 80% of mineral is accreted during the third trimester; calcium transfers at 300-350 mg/day during the final 6 wk. The neonate requires 200 mg calcium daily from milk during the first 6 mo, and 120 mg calcium from milk during the second 6 mo (additional calcium comes from solid foods). Calcium transfers can be more than double and triple these values, respectively, in women who nurse twins and triplets. About 25% of dietary calcium is normally absorbed in healthy adults. Average maternal calcium intakes in American and Canadian women are insufficient to meet the fetal and neonatal calcium requirements if normal efficiency of intestinal calcium absorption is relied upon. However, several adaptations are invoked to meet the fetal and neonatal demands for mineral without requiring increased intakes by the mother. During pregnancy the efficiency of intestinal calcium absorption doubles, whereas during lactation the maternal skeleton is resorbed to provide calcium for milk. This review addresses our current knowledge regarding maternal adaptations in mineral and skeletal homeostasis that occur during pregnancy, lactation, and post-weaning recovery. Also considered are the impacts that these adaptations have on biochemical and hormonal parameters of mineral homeostasis, the consequences for long-term skeletal health, and the presentation and management of disorders of mineral and bone metabolism.

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Maternal Mineral and Bone Metabolism During Pregnancy, Lactation, and Post-Weaning Recovery

During pregnancy, the mother adapts to meet the calcium demands of the fetus. The effect of this adaptation on the maternal skeleton is not fully understood. Our objectives were to evaluate changes in bone mineral density (BMD) and bone turnover during pregnancy. We studied 16 women longitudinally, with baseline measurements before pregnancy; then at 16, 26, and 36 weeks of pregnancy; and postpartum. We measured total-body BMD and biochemical markers of bone resorption (urinary pyridinium crosslinks and telopeptides of type I collagen) and bone formation (serum bone alkaline phosphatase, propeptides of type I procollagen [PINP] and osteocalcin). We also measured parathyroid hormone (PTH), insulin-like growth factor I (IGF-I), and human placental lactogen. Postpartum, BMD increased in the arms (2.8%, P < 0.01) and legs (1.9%, P < 0.01) but decreased in the pelvis (-3.2%, P < 0.05) and spine (-4.6%, P < 0.01) compared with prepregnancy values. All biochemical markers, with the exception of osteocalcin concentration, increased during pregnancy. The change in IGF-I at 36 weeks was related to the change in biochemical markers (e.g., PINP, r = 0.72, P = 0.002). Pregnancy is a high-bone-turnover state. IGF-I levels may be an important determinant of bone turnover during pregnancy. Elevated bone turnover may explain trabecular bone loss during pregnancy.

The effect of pregnancy on bone density and bone turnover.

We measured serum alkaline phosphatase isoenzymes and osteocalcin levels in 40 healthy women at 4-week intervals throughout uncomplicated pregnancies and 6 weeks after delivery in 17 women. Serum bone alkaline phosphatase was significantly higher in the third trimester than in early pregnancy (P < 0.001), and this elevation was still apparent at the end of the puerperium, suggesting increased bone turnover. Serum osteocalcin was not detected (<0.2 μg/L) after the first trimester in the majority of women, and it reappeared within 48 h after delivery. The disappearance of osteocalcin after the first trimester and its rapid reappearance after delivery suggest placental clearance of this peptide. We conclude that serum osteocalcin measurements cannot be used as a marker of bone metabolism during pregnancy.

Serum concentrations of alkaline phosphatase isoenzymes and osteocalcin in normal pregnancy.

SummaryAlterations in thyroid function during normal pregnancy were investigated in this study. Serial blood samples were taken from 40 healthy women throughout their pregnancies and a further specimen was obtained 6 weeks after delivery (17 cases). The following assays were performed: free and total T, and T4, rT3, TSH and thyroid binding globulin. Changes in these are discussed with reference to interpretation of biochemical tests of thyroid function in pregnancy.

G. Mashiter

Papers

Serum concentrations of alkaline phosphatase isoenzymes and osteocalcin in normal pregnancy.

Thyroid function in normal pregnancy