Table 14-1summarizes the main physiologic changes that occur during a normal pregnancy, and which relate to thyroid function or thyroid function testing. Early in pregnancy there is an increase in renal blood flow and glomerular filtration which lead to an increase in iodide clearance from plasma [ 2-6]. This results in a fall in plasma iodine concentrations and an increase in the iodide requirements from the diet [2]. In women with iodine sufficiency there is little impact, in terms of thyroid function, of the obligatory increase in renal iodine losses, because the intrathyroidal iodine stores are plentiful at the time of conception and remain unaltered throughout gestation. As an example, in a collaborative study between the Universities of Massachusetts (USA) and Santiago (Chili), iodine metabolism was investigated in the three trimesters of gestation and again after delivery. Plasma inorganic iodide (PII) concentrations, urinary iodide levels, and thyroid function tests were determined in 16 pregnant women. The results showed a wide variability in PII values and urinary iodide concentrations, but no trend for a decrease in PII concentrations during pregnancy. The conclusion was that pregnancy does not have a major influence on circulating iodine concentrations in iodine-sufficient regions. It should be noted, however, that the iodine excretion levels were unusually high in this study, ranging between 459-786 μg/day [7].

In regions where the iodine supply is borderline or low, the situation is clearly different and significant changes occur during pregnancy [ 3, 4, 6]. While 24-hour radioiodine uptake determinations are not usually performed in the pregnant state, past studies have shown that the uptake is increased [8]. In addition, there is a further increment in the iodine requirements, due to transplacental iodide transport necessary for iodothyronine synthesis by the fetal thyroid gland, which becomes progressively functional after the first trimester. Several studies have convincingly documented the notion that, when a pregnancy takes place in conditions with borderline iodine availability, significant increments in both maternal and fetal thyroid volume occur, when no supplemental iodine was given during early pregnancy [ 9-11]. This is in sharp contrast with the established fact that in iodine-sufficient areas, there is little, if any, change in thyroid size during pregnancy [12, 13].

Human chorionic gonadotropin (hCG) is a member of the glycoprotein hormone family that is composed of a common α-subunit and a non-covalently associated, hormone-spcific β-subunit. The α-subunit of hCG consists of a polypeptide chain of 92 amino acid residues containing two N-linked oligosaccharide side-chains. The β-subunit of hCG consists of 145 residues with two N-linked and four O-linked oligosaccharide side-chains. The β-subunit of TSH is composed of 112 residues and one N-linked oligosaccharide. The β-subunits of both molecules possess 12 half-cysteine residues at highly conserved positions. Three disulfide bonds form a cystine knot structure, which is identical in both TSH and hCG and is essential for binding to their receptor (LH and hCG bind to the same receptor, the LHCG receptor). A single gene on chromosome 6 encodes for the common α-subunit, while the genes that encode for the β-subunits are clustered on chromosome 19, with seven genes (but only three actively transcribed) coding for β-hCG [14-16].

The structural homology between hCG and TSH provides already an indication that hCG may act as a thyrotropic agonist, by overlap of their natural functions. Human CG possesses an intrinsic (albeit weak) thyroid-stimulating activity and perhaps even a direct thyroid-growth-promoting activity [17-24]. During normal pregnancy, the direct stimulatory effect of hCG on thyrocytes induces a small and transient increase in free thyroxine levels near the end of the 1st trimester (peak circulating hCG) and, in turn, a partial TSH suppression [ 3, 6, 25-31]. When tested in bioassays, hCG is only about 1/104 as potent as TSH during normal pregnancy. This weak thyrotropic activity explains why, in normal conditions, the effects of hCG remain largely unnoticed and thyroid function tests mostly unaltered.

The thyrotropic role of hCG in normal pregnancy is illustrated in Figure 14-1. The figure shows the inverse relationship between serum hCG and TSH concentrations, with a mirror image between the nadir of serum TSH and peak hCG levels et the end of the first trimester. The inset in the figure shows that the rise in serum free T4 is proportional to peak hCG values. At this period during gestation, 1/5th of otherwise euthyroid pregnant women have a transiently lowered serum TSH, even below the lower limit of the normal non pregnant reference range [ 3, 24, 32].

Figure 1. The pattern of serum TSH and hCG changes are shown as a function of gestation age in 606 healthy pregnant women. Between 8 and 14 weeks gestation, changes in serum hCG and TSH are mirror images of each other, and there is a significant negative correlation between the individual TSH (nadir) and peak hCG levels (P<0.001) (hCG: ▲-----▲ ; TSH: ●-----●). The inset shows a scattergram of serum free T4 levels in the same women plotted in relation to circulating hCG concentrations (by 10.000 IU/L increment) during the first half of gestation. The figure shows the direct relationship between free T4 and hCG, with progressively increasing free T4 levels (from Glinoer, Ref 3).

Experimental studies with desialylated and deglycosylated hCG, using T3 secretion as the response parameter (in a serum-free culture system with human thyroid follicles), have shown that removal of sialic acid or carbohydrate residues from native hCG transformed such hCG variants into thyroid stimulating super-agonists [33]. Further evidence to support the patho-physiological role of hCG to stimulate excessively the human thyroid gland is can be found in studies of patients with hydatidiform mole and choriocarcinoma (see Chapter 13). In these conditions, clinical and biochemical manifestations of hyperthyroidism often occur and, as expected, the abnormal stimulation of the thyroid is rapidly relieved after appropriate surgical treatment [34-36].

Vomiting occurs in normal pregnancy during the 1st trimester and usually ceases by the 15th week. Prolonged nausea and severe vomiting in early pregnancy that causes greater than a 5% weight loss, dehydration and ketonuria is defined as Hyperemesis Gravidarum (HG) and occurs in 0.5-10 cases per 1,000 pregnancies [37]. Hyperemesis is associated with high hCG levels occurring at this time, but the exact cause remains uncertain. For unknown reasons, HG seems to be more prevalent in Asian than Caucasian women. Thirty to sixty percent of patients with HG have elevations of serum free thyroid hormone concentrations with a suppressed TSH [38]. Women with hyperemesis and elevated thyroid hormone levels most commonly do not have other clinical evidence of primary thyroid disease, such as Graves’ disease. A minor proportion of these patients may have clinical hyperthyroidism, termed ‘gestational hyperthyroidism’ or ‘gestational transient thyrotoxicosis’ (GTT). Obviously, Graves’ disease can also occur coincident with hyperemesis [39]. Finally, many common signs and symptoms of hyperthyroidism may be mimicked by a normal pregnancy, and the clinical challenge is to differentiate these disorders [40-48].

The etiology of excessive thyroid stimulation is considered to be hCG itself (or derivatives of hCG) via a direct stimulation of the thyroid cells through binding of hCG to the TSH receptor [49, 50]. In virtually all patients with gestational hyperthyroidism, appropriate fluid replacement will lead to resolution of the clinical symptoms. As gestation proceeds and hCG levels progressively fall, normal thyroid function is resumed. In severe (but rare) cases, antithyroid drug treatment may be required (described in more detail below). Several investigators have observed that there may even be more subtle form of hyperthyroidism associated with morning sickness [ 41, 42, 46]. Severity of emesis was correlated with serum free T4 and hCG levels (and inversely with the degree of TSH suppression), suggesting strongly that HG may reflect the extreme end of the spectrum of physiological changes that occur at this time in normal pregnancy ( Figure 14-2). One tempting hypothesis to correlate conceptually the two is to consider that high hCG levels cause both an increased estrogen secretion as well as thyroid hyperfunction, and in turn explain the coexistence of nausea and vomiting with hyperthyroidism [45].

Figure 2. Relationship between the severity of vomiting and the mean (with SE) serum concentrations of hCG, free T4, and TSH. The inset in the lower right part of the figure shows the prevalence of suppressed TSH levels, for each trimester of gestation, in a cohort of normal pregnant women. The data were graphically adapted by Carole Spencer (with my thanks to Carole for allowing me to borrow the slide). The figures are based on studies by Goodwin (Ref 42) & Glinoer (Ref 24).

The charge-isoforms profiles of circulating hCG were investigated in women hyperemesis gravidarum (HG) and different ethnic backgrounds (Samoan vs European) [51]. The results confirmed an increase in total serum hCG concentrations as well as an increase in the proportion of acidic hCG variants in the women suffering from HG, compared with matched control subjects. The same study also confirmed the known association between hCG concentrations in early pregnancy and elevations in thyroid hormone levels [6] While there was no major association between HG and ethnic background, the authors observed a high prevalence of recurrent HG and a familial predisposition for this condition, suggesting that either long-term environmental factors or genetic factors may play a crucial role in the pathogenesis of HG and gestational transient non autoimmune thyrotoxicosis.

The increase in total serum T4 and T3 that occurs during pregnancy is due to an increase in serum thyroxine binding globulin (TBG) concentrations. Changes in TBG happen early and, by 16-20 weeks of gestation, TBG concentrations have doubled [ 3, 6]. The cause of the marked increase in serum TBG is probably multifactorial. Earlier studies showed that TBG biosynthesis was increased, after estradiol priming, in primary cultures of hepatocytes from Rhesus monkeys [52]. However, the lack of increase in other binding proteins (CBG & SHBG) by estrogen in HEP-G2 cells raised the possibility that other factors might be operative in the pregnant state. Studies of changes in the glycosylation patterns of TBG, induced by estrogen, have indicated

that the increase in circulating levels of TBG was due in large part to a reduction of its plasma clearance (see also Chapters 3, 5) [53]. Sera of pregnant or estrogen-treated individuals show a marked increase in the more heavily sialylated fractions of TBG. This increase in the sialic acid content of TBG inhibits the uptake of the protein by specific asialylo-glycoprotein receptors on hepatocytes, and the more heavily sialylated proteins from pregnant sera have therefore a longer plasma half-life [54]. Such alterations in sialylation are not found in TBG isolated from patients with congenital TBG elevation, the latter being due to a true over-production of the protein [55]. Thus, in addition to the stimulatory estrogen effects of estrogen on TBG synthesis, a major contribution to the increased TBG concentration during pregnancy is the reduced clearance of the protein. This explanation is attractive since it would also also account for the increases observed in concentrations of other circulating glycoproteins in hyper-estrogenic states. Delivery leads to a rapid reversal of this process and serum TBG concentrations return to normal within 4-6 weeks. With that, serum T4 and T3 also return to pregestational serum levels. In addition to the 2 to 3-fold increase in serum TBG, modest decreases in both serum transthyretin (TTR) and albumin are commonly found in pregnancy, but the physiological impact of these changes, if any, is unknown [56].

An interesting case report was published by Zigman et al. in 2003 [57]. A 42-year-old woman had both established hypothyroidism and inherited TBG deficiency, and was followed by the authors through 2 full-term pregnancies. The patient had a baseline TBG level that was approximately 70% below the average baseline level of non-TBG-deficient women. During her pregnancies, serum TBG levels rose, although remaining at only one half the usual increment in TBG associated with normal pregnancy. Despite the patient’s low baseline TBG level and blunted pregnancy-associated TBG rise, she required an increase in her thyroxine replacement doses that mirrored those observed in hypothyroid, but non-TBG-deficient pregnant women. The authors suggested therefore that an increase in TBG concentration was not the key determinant for the increase in thyroxine requirement in pregnancy. In a letter to the Editor, an alternative explanation was proposed [58]. In the normal situation before pregnancy, the homeostasis of thyroid function is ensured by the equilibrium between a serum total T4 of ~100 nmol/L and a TBG concentration of ~260 nmol/L. This equilibrium implies, in turn, that ~75 % of the circulating T4 is bound to TBG and that ~35-40 % of circulating TBG is saturated by T4. During a normal pregnancy, the extracellular TBG pool expands from ~3,000 to ~7,000 nmol/L. Thus, for the homeostasis of free thyroid hormones to be maintained, the extra-thyroidal total thyroxine pool must parallel this expansion, and this can only be achieved by the thyroid gland filling up the progressively the increased hormonal pool during the first half of pregnancy (see Figure 14-3). In the exceptional case of Zigman, when this partially TBG-deficient patient was not pregnant, her serum total T4 was ~70 nmol/L and TBG ~80 nmol/L, indicating that her circulating TBG was almost completely saturated by T4, because of her severe restriction in the TBG binding capacity. However in the non pregnant condition, only a relatively small fraction of the patient’s circulating T4 could be bound to TBG: ~50%. When the patient became pregnant, her TBG deficiency was still partially responsive to estrogen induction and TBG increased 3-fold to ~240 nmol/L and total T4 to ~90 nmol/L. In other words, her total T4 concentrations had to be raised by ~30% (via an increase in thyroxine replacement), hence allowing to restore a TBG binding saturation level by T4 of ~35%, equivalent to what is observed at the onset of pregnancy in non-TBG-deficient women. Thus, the increment required in l-T4 dosage was precisely of the same proportion than that anticipated from the partial rise in serum TBG during pregnancy.

Figure 3a. This panel illustrates the rapid changes that occur in serum total binding capacity of TBG during the first half of gestation under the influence of elevated estrogen levels.

Figure 3b. This panel shows that, in order to maintain unaltered free T4 levels, the markedly increased TBG extra-cellular pool must steadily be filled with increasing amounts of T4, until a new equilibrium is reached. This is achieved during pregnancy via an overall ~50% increase in thyroid hormone production.

The increased plasma concentration of TBG, together with the increased plasma volume, results in a corresponding increase in the total T4 pool during pregnancy. While the changes in TBG are most dramatic during the first trimester, the increase in plasma volume continues until delivery. Thus, for free T4 concentration to remain unaltered, the T4 production rate must increase (or its degradation rate decrease) to allow for additional T4 to accumulate. One would predict that in a situation where the T4 input was constant, there would be an iterative increment in T4 as TBG increases, due to reduced T4 availability to degradation enzymes. The evidence that thyroxine requirements are markedly enhanced during pregnancy in hypothyroid treated women (see section on maternal hypothyroidism) strongly suggests that not only T4 degradation is decreased in early pregnancy but also that an increased T4 production occurs throughout gestation to maintain the homeostasis of free T4 concentrations [6, 59-62].

The only direct measurements of T4 turnover rates in pregnancy were obtained more than 30 years ago by Dowling et al [63]. In eight pregnant subjects (4 in 1st half & 4 in 2nd half of gestation), T4 turnover rates were estimated not to be significantly different from those of non-pregnant subjects. However, based on several considerations discussed above from more recent work, it can now be concluded that the T4 production rate is enhanced during pregnancy. Globally, it is accepted that there is a ~50 % increase in the production of T4 during gestation [6, 64].

The placenta contains high concentrations of Type 3 or inner-ring (5) iodothyronine deiodinase [75-77]. Inner-ring deiodination of T4 catalyzed by this enzyme is the source of high concentrations of reverse T3 present in amniotic fluid, and the reverse T3 levels parallel maternal serum T4 concentrations [78-81]. This enzyme may function to reduce the concentrations of T3 and T4 in the fetal circulation (the latter being still contributed by 20-30 % from thyroid hormones of maternal origin at the time of parturition), although fetal tissue T3 levels can reach adult levels due to the local activity of the Type 2 deiodinase (see Chapter 15) [59]. Type 3 deiodinase may also indirectly provide a source of iodide to the fetus via iodothyronine deiodination. Despite the presence of placental Type 3 deiodinase, in circumstances in which fetal T4 production is reduced or maternal free T4 markedly increased, transplacental passage occurs and fetal serum T4 levels are about one third of normal [82]. Thyroxine can be detected in amniotic fluid prior to the onset of fetal thyroid function, indicating its maternal origin by transplacental transfer [83]. Figure 14-6 depicts the steep maternal to fetal gradient of total T4 concentrations in early pregnancy stages. Between 6-12 weeks gestation, if maternal total T4 concentration is set to represent 100%, the total T4 concentration in the coelomic fluid would represent 0.07% and T4 in the amniotic cavity as little as 0.0003-0.0013% of maternal total T4 concentrations. Thus, the placental barrier to maternal iodothyronines is not impermeable to the transplacental passage of thyroid hormones of maternal origin, even in the 3rd trimester [59, 84]. Even though very small quantitatively, such concentrations may qualitatively represent an extremely important source of thyroid hormones to ensure the adequate development of the feto-maternal unit [85, 86].

Figure 6. Steep gradient between maternal concentrations of thyroid hormones and those measured in the coelomic fluid and amniotic cavity with the developing embryo, during early stages of gestation.