Gregory A. Brent

Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and Postpartum

Pregnancy has a profound impact on the thyroid gland and thyroid function. The gland increases 10% in size during pregnancy in iodine-replete countries and by 20%– 40% in areas of iodine deficiency. Production of thyroxine (T4) and triiodothyronine (T3) increases by 50%, along with a 50% increase in the daily iodine requirement. These physiological changes may result in hypothyroidism in the later stages of pregnancy in iodine-deficient women who were euthyroid in the first trimester. The range of thyrotropin (TSH), under the impact of placental human chorionic gonadotropin (hCG), is decreased throughout pregnancy with the lower normal TSH level in the first trimester being poorly defined and an upper limit of 2.5 mIU/L. Ten percent to 20% of all pregnant women in the first trimester of pregnancy are thyroid peroxidase (TPO) or thyroglobulin (Tg) antibody positive and euthyroid. Sixteen percent of the women who are euthyroid and positive for TPO or Tg antibody in the first trimester will develop a TSH that exceeds 4.0 mIU/L by the third trimester, and 33%–50% of women who are positive for TPO or Tg antibody in the first trimester will develop postpartum thyroiditis. In essence, pregnancy is a stress test for the thyroid, resulting in hypothyroidism in women with limited thyroidal reserve or iodine deficiency, and postpartum thyroiditis in women with underlying Hashimoto’s disease who were euthyroid prior to conception. Knowledge regarding the interaction between the thyroid and pregnancy/the postpartum period is advancing at a rapid pace. Only recently has a TSH of 2.5 mIU/L been accepted as the upper limit of normal for TSH in the first trimester. This has important implications in regards to interpretation of the literature as well as a critical impact for the clinical diagnosis of hypothyroidism. Although it is well accepted that overt hypothyroidism and overt hyperthyroidism have a deleterious impact on pregnancy, studies are now focusing on the potential impact of subclinical hypothyroidism and subclinical hyperthyroidism on maternal and

Mechanisms of thyroid hormone action

Our understanding of thyroid hormone action has been substantially altered by recent clinical observations of thyroid signaling defects in syndromes of hormone resistance and in a broad range of conditions, including profound mental retardation, obesity, metabolic disorders, and a number of cancers. The mechanism of thyroid hormone action has been informed by these clinical observations as well as by animal models and has influenced the way we view the role of local ligand availability; tissue and cell-specific thyroid hormone transporters, corepressors, and coactivators; thyroid hormone receptor (TR) isoform-specific action; and cross-talk in metabolic regulation and neural development. In some cases, our new understanding has already been translated into therapeutic strategies, especially for treating hyperlipidemia and obesity, and other drugs are in development to treat cardiac disease and cancer and to improve cognitive function.

Levothyroxine Therapy in Patients with Thyroid Disease

Levothyroxine is one of the 13 most commonly prescribed medications in the United States, with more than 15 million prescriptions filled annually [1]. It is given as either physiologic replacement therapy in patients with hypothyroidism or as interventional therapy to suppress thyroid-stimulating hormone (TSH) secretion in patients with nodular thyroid disease or thyroid cancer. Overt hypothyroidism occurs in 1.5% to 2% of women and in 0.2% of men [2], and its incidence increases with age; among persons older than 60 years, 6% of women and 2.5% of men have serum TSH levels greater than twice the upper limit of normal [3]. Thyroid nodules are present in 0.8% of men but occur in 5% of women and increase in incidence after 45 years of age [2]. Thyroid cancer is the most common endocrine cancer, with an annual incidence of 10 000 new cases in the United States [4]. Measurement of serum TSH permits precise levothyroxine dosage, both in replacement therapy, where serum concentrations are maintained in the normal range, and in TSH suppressive therapy, where supraphysiologic doses of levothyroxine are given to maintain the serum TSH concentration below normal. In addition to suppressing serum TSH, excess levothyroxine administration is associated with other signs of thyrotoxicosis at the tissue level. Decreased bone mineral density [5-8] and an accelerated rate of bone loss [9] have been reported in both pre- and postmenopausal women receiving sufficient levothyroxine to yield subnormal serum TSH levels. Consequently, excess levothyroxine therapy, either intentional or inadvertent, is not as innocuous as was once supposed, at least for women. Unintentional over-replacement is still a common problem. In several recent studies, as many as 50% of patients receiving levothyroxine replacement therapy who were clinically euthyroid were actually overtreated, as judged by their suppressed serum TSH concentrations [10, 11]. Furthermore, TSH assays sufficiently sensitive to permit accurate quantitation of subnormal values are, unfortunately, still not routine in certain areas of the United States [12]. We review the physiologic, pharmacologic, and therapeutic aspects of therapy with levothyroxine. Because the benefit-risk considerations are distinctly different for replacement and interventional therapy, these are discussed separately. For some patients with nodular thyroid disease, conflicting or insufficient data may not allow an unequivocal consensus recommendation. In such circumstances, we review the existing evidence and provide a rational strategy for treatment. Background Thyroid Hormone Physiology and Thyroid-Pituitary Regulation Thyroxine (T4) and 3,5,3 triiodothyronine (T3), so named because of the number of iodide atoms bound to a thyronine backbone, are the main circulating thyroid hormones. Thyroxine is produced only by the thyroid. However, in euthyroid humans, only 20% of circulating T3 is secreted by the thyroid and 80% is produced in extra-thyroidal tissue by monodeiodination of T4. About 40% of the T4 is deiodinated to T (3) in the liver and kidney [13] by the recently cloned type I deiodinase, a selenoprotein [14, 15]. The presence of selenocysteine renders this T4 to T3 conversion process sensitive to levels of dietary selenium as well as to inhibition by propylthiouracil [16]. Illness, caloric deprivation, and various drugs and radiographic contrast agents also inhibit deiodination of T4 to T3 [13]. In addition, T4 is metabolized by other pathways not resulting in T3 generation, such as sulfation and inner-ring deiodination or glucuronidation leading to biliary excretion. It is T3 that enters the cell nucleus, binds to its nuclear receptor, and regulates transcription of thyroid hormone-responsive genes, resulting in the physiologic changes associated with thyroid hormone [17]. To understand the effects of T4 and its therapeutic use, it is important to appreciate the differences in the quantity and sources of T3 in the nuclei of different tissues. Nearly all of nuclear T3 present in the liver, kidney, skeletal muscle, and heart of euthyroid rats originates from serum T3. However, the pituitary and central nervous system differ in that local conversion of T4 to T3 by the type II deiodinase within these tissues contributes equally (pituitary) or as much as 80% (cerebral cortex) to the nuclear-bound T3 (these studies are reviewed in reference 13). Thus, although most peripheral tissues depend primarily on circulating T3, the pituitary and central nervous system are sensitive to both circulating T4 and T3 [18-21]. These concepts are summarized in Figure 1. Figure 1. Role of thyroxine (T4) and 3,5,3 triiodothyronine (T3) in the feedback regulation of thyrotropin-releasing hormone (TRH) and thyroid-stimulating hormone (TSH) secretion. Thyroid-Stimulating Hormone Assay Because of the precise feedback relation between circulating thyroid hormones and pituitary TSH secretion, measurements of serum TSH concentrations are essential in the management of patients receiving levothyroxine therapy. The limit of detection of the new immunometric assays currently in use is at least 10 times lower than that of the original radioimmunoassays, and the new assays can reliably distinguish between normal and suppressed serum TSH concentrations [22-25]. Furthermore, the basal serum TSH concentration is proportional to the TSH response to thyrotropin-releasing hormone (TRH), making TRH stimulation tests obsolete [22, 26]. The only caveat to this generalization concerns patients with central hypothyroidism. Because the current strategy for monitoring levothyroxine therapy depends so heavily on TSH measurement, it is also important to recognize those factors, independent of thyroid hormones, that influence TSH secretion (Table 1). For the purposes of this review, the commonly accepted normal range of 0.5 to 5.0 mU/L is used for serum TSH. Table 1. Situations Associated with Thyroid-Stimulating Hormone Suppression Pharmacologic Aspects of Levothyroxine Preparations Levothyroxine is synthetically produced but identical to T4 secreted by the thyroid. According to the United States Pharmacopeia, the T4 content of tablets must be between 90% and 110% of the stated amount, as measured by high pressure liquid chromatography [27]. The gastrointestinal absorption of levothyroxine is approximately 80% [28] and does not differ in the hypothyroid or euthyroid state [29]. Absorption occurs along the entire human small intestine, but the rapidity decreases distally [30, 31]. After ingestion of levothyroxine, serum T4 levels peak at 2 to 4 hours (average increase, 10% to 15% over basal concentrations) and remain above this basal level for up to 6 hours [32, 33]. The increase in serum T3 levels after levothyroxine administration is slow because of the time required for T4 to T3 conversion. Serum T3 concentrations are generally stable during maintenance levothyroxine therapy, unlike after administration of triiodothyronine itself [34]. Several proprietary and multiple generic levothyroxine preparations are available. Many formulations are marketed in several convenient strengths (25, 50, 75, 88, 100, 112, 125, 150, 175, 200, and 300 g) that allow precise dosage with a single tablet, enhancing compliance. Changes in Levothyroxine Requirements Several conditions or drugs may alter levothyroxine requirements for both replacement and interventional therapy (Table 2). The serum TSH level should be monitored more often in these circumstances, and the levothyroxine dose should be adjusted to maintain the serum TSH level in the normal range for hypothyroid patients receiving replacement therapy or at its appropriate therapeutic level for those receiving suppressive therapy. During pregnancy, the average increase in levothyroxine requirements is 45%, and monitoring should be carried out every 2 months [35, 36]. After delivery, the dose can be immediately reduced to the prepregnancy level and the serum TSH level should be measured again 6 weeks after delivery. Patients should be instructed to take levothyroxine at least 3 hours after the administration of medications that may interfere with its intestinal absorption (Table 2). In general, the increase in the levothyroxine requirement will be less than twofold [43, 46]. Thyroid function tests should be checked 1 month after the initiation of therapy with such agents, and the levothyroxine dose should be adjusted accordingly. If the drug is then withdrawn, the levothyroxine dosage may have to be decreased. Rifampin [42], carbamazepine [43], and possibly phenytoin [44] accelerate T4 clearance via pathways that do not lead to T3 production, thereby increasing levothyroxine requirements. Amiodarone competitively inhibits T4 to T3 conversion [45]. Table 2. Circumstances in Which Levothyroxine Requirements May Be Altered* Physiologic Effects of Excessive Levothyroxine Administration There is growing evidence that excess levothyroxine administration, as defined by a suppressed serum TSH concentration, is associated with physiologic alterations in peripheral tissue. Cardiac changes include shortening of systolic time intervals [49, 50] and increases in nocturnal heart rate [51]. Hepatic enzymes (alanine aminotransferase) and proteins (sex hormone-binding globulin, ferritin) may also be increased [52, 53], and red blood cell ouabain binding is decreased [54]. Thyroid hormone accelerates bone turnover. Both serum osteocalcin, a marker of this turnover, and urinary pyridinium cross-link excretion, a more specific indicator of bone resorption, are increased in women receiving supraphysiologic levothyroxine doses [55, 56]. Premenopausal women treated with excess levothyroxine show predominantly cortical bone loss, measured in the wrist and hip, as opposed to trabecular bone loss, measured in the spine [5-7]. Similarly treated postmenopausal women show reductions in both trabecular and cortical bone mineral density [7-9]. T

A thyroid hormone receptor α gene mutation (P398H) Is associated with visceral adiposity and impaired catecholamine-stimulated lipolysis in mice

Thyroid hormone has profound effects on metabolic homeostasis, regulating both lipogenesis and lipolysis, primarily by modulating adrenergic activity. We generated mice with a point mutation in the thyroid hormone receptor α (TRα) gene producing a dominant-negative TRα mutant receptor with a proline to histidine substitution (P398H). The heterozygous P398H mutant mice had a 3.4-fold (p < 0.02) increase in serum thyrotropin (TSH) levels. Serum triiodothyronine (T3) and thyroxine (T4) concentrations were slightly elevated compared with wild-type mice. The P398H mice had a 4.4-fold increase in body fat (as a fraction of total body weight) (p < 0.001) and a 5-fold increase in serum leptin levels (p < 0.005) compared with wild-type mice. A 3-fold increase in serum fasting insulin levels (p < 0.002) and a 55% increase in fasting glucose levels (p < 0.01) were observed in P398H compared with wild-type mice. There was a marked reduction in norepinephrine-induced lipolysis, as reflected in reduced glycerol release from white adipose tissue isolated from P398H mice. Heart rate and cold-induced adaptive thermogenesis, mediated by thyroid hormone-catecholamine interaction, were also reduced in P398H mice. In conclusion, the TRα P398H mutation is associated with visceral adiposity and insulin resistance primarily due to a marked reduction in catecholamine-stimulated lipolysis. The observed phenotype in the TRα P398H mouse is likely due to interference with TRα action as well as influence on other metabolic signaling pathways. The physiologic significance of these findings will ultimately depend on understanding the full range of actions of this mutation.

Ectopic expression of the thyroperoxidase gene augments radioiodide uptake and retention mediated by the sodium iodide symporter in non-small cell lung cancer.

Radioiodide is an effective therapy for thyroid cancer. This treatment modality exploits the thyroid-specific expression of the sodium iodide symporter ( NIS ) gene, which allows rapid internalization of iodide into thyroid cells. To test whether a similar treatment strategy could be exploited in nonthyroid malignancies, we transfected non–small cell lung cancer (NSCLC) cell lines with the NIS gene. Although the expression of NIS allowed significant radioiodide uptake in the transfected NSCLC cell lines, rapid radioiodide efflux limited tumor cell killing. Because thyroperoxidase ( TPO ) catalyzes iodination of proteins and subsequently causes iodide retention within thyroid cells, we hypothesized that coexpression of both NIS and TPO genes would overcome this deficiency. Our results show that transfection of NSCLC cells with both human NIS and TPO genes resulted in an increase in radioiodide uptake and retention and enhanced tumor cell apoptosis. These findings suggest that single gene therapy with only the NIS gene may have limited efficacy because of rapid efflux of radioiodide. In contrast, the combination of NIS and TPO gene transfer, with resulting TPO-mediated organification and intracellular retention of radioiodide, may lead to more effective tumor cell death. Thus, TPO could be used as a therapeutic strategy to enhance the NIS-based radioiodide concentrator gene therapy for locally advanced lung cancer. Cancer Gene Therapy (2001) 8, 612–618

Thyroid hormone crosstalk with nuclear receptor signaling in metabolic regulation

Thyroid hormone influences diverse metabolic pathways important in lipid and glucose metabolism, lipolysis and regulation of body weight. Recently, it has been recognized that thyroid hormone receptor interacts with transcription factors that predominantly respond to nutrient signals including the peroxisome proliferator-activated receptors, liver X receptor and others. Crosstalk between thyroid hormone signaling and these nutrient responsive factors occurs through a variety of mechanisms: competition for retinoid X receptor heterodimer partners, DNA binding sites and transcriptional cofactors. This review focuses on the mechanisms of interaction of thyroid hormone signaling with other metabolic pathways and the importance of understanding these interactions to develop therapeutic agents for treatment of metabolic disorders, such as dyslipidemias, obesity and diabetes.

Expression of Uncoupling Protein 1 in Mouse Brown Adipose Tissue Is Thyroid Hormone Receptor-β Isoform Specific and Required for Adaptive Thermogenesis

Cold-induced adaptive (or nonshivering) thermogenesis in small mammals is produced primarily in brown adipose tissue (BAT). BAT has been identified in humans and becomes more active after cold exposure. Heat production from BAT requires sympathetic nervous system stimulation, T(3), and uncoupling protein 1 (UCP1) expression. Our previous studies with a thyroid hormone receptor-beta (TR beta) isoform-selective agonist demonstrated that after TR beta stimulation alone, adaptive thermogenesis was markedly impaired, although UCP-1 expression in BAT was normal. We used mice with a dominant-negative TR beta PV mutation (frameshift mutation in resistance to thyroid hormone patient PV) to determine the role of TR beta in adaptive thermogenesis and UCP1 expression. Wild-type and PV mutant mice were made hypothyroid and replaced with T(3) (7 ng/g x d) for 10 d to produce similar serum thyroid hormone concentration in the wild-type and mutant mice. The thermogenic response of interscapular BAT, as determined by heat production during iv infusions of norepinephrine, was reduced in PV beta heterozygous and homozygous mutant mice. The level of UCP1, the key thermogenic protein in BAT, was progressively reduced in PV beta(+/-) and PV beta(-/-) mutant mice. Brown adipocytes isolated from PV mutant mice had some reduction in cAMP and glycerol production in response to adrenergic stimulation. Defective adaptive thermogenesis in TR beta PV mutant mice is due to reduced UCP1 expression and reduced adrenergic responsiveness. TR beta mediates T(3) regulation of UCP1 in BAT and is required for adaptive thermogenesis.

The Sodium Iodide Symporter (NIS): Regulation and Approaches to Targeting for Cancer Therapeutics

Expression of the sodium iodide symporter (NIS) is required for efficient iodide uptake in thyroid and lactating breast. Since most differentiated thyroid cancer expresses NIS, β-emitting radioactive iodide is routinely utilized to target remnant thyroid cancer and metastasis after total thyroidectomy. Stimulation of NIS expression by high levels of thyroid-stimulating hormone is necessary to achieve radioiodide uptake into thyroid cancer that is sufficient for therapy. The majority of breast cancer also expresses NIS, but at a low level insufficient for radioiodine therapy. Retinoic acid is a potent NIS inducer in some breast cancer cells. NIS is also modestly expressed in some non-thyroidal tissues, including salivary glands, lacrimal glands and stomach. Selective induction of iodide uptake is required to target tumors with radioiodide. Iodide uptake in mammalian cells is dependent on the level of NIS gene expression, but also successful translocation of NIS to the cell membrane and correct insertion. The regulatory mechanisms of NIS expression and membrane insertion are regulated by signal transduction pathways that differ by tissue. Differential regulation of NIS confers selective induction of functional NIS in thyroid cancer cells, as well as some breast cancer cells, leading to more efficient radioiodide therapy for thyroid cancer and a new strategy for breast cancer therapy. The potential for systemic radioiodide treatment of a range of other cancers, that do not express endogenous NIS, has been demonstrated in models with tumor-selective introduction of exogenous NIS.

Environmental Exposures and Autoimmune Thyroid Disease

BACKGROUND Environmental exposures, ranging from perchlorate in rocket fuel to polychlorinated biphenols, have been shown to influence thyroid function. Although most of these agents are associated with reduced thyroid hormone levels or impaired thyroid hormone action, a number of environmental exposures confer an increased risk of autoimmune thyroid disease. SUMMARY Factors that increase autoimmune thyroid disease risk include radiation exposure, both from nuclear fallout and medical radiation, increased iodine intake, as well as several contaminants in the environment that influence the thyroid. Although approximately 70% of the risk for developing autoimmune thyroid disease is attributable to genetic background, environmental triggers are thought to play a role in the development of autoimmune thyroid disease in susceptible individuals. CONCLUSIONS Understanding the association of environmental agents with thyroid dysfunction can be utilized to reduce the risk to populations. Knowledge of the specific factors that trigger autoimmune thyroid disease and their mode of action, however, may also inform risk reduction in the individual patient. These factors are especially relevant for those at increased risk of autoimmune thyroid disease based on family history.

Phosphoinositide-3-kinase inhibition induces sodium/iodide symporter expression in rat thyroid cells and human papillary thyroid cancer cells.

TSH stimulation of sodium iodide symporter (NIS) expression in thyroid cancer promotes radioiodine uptake and is required to deliver an effective treatment dose. Activation of the insulin/phosphoinositide-3-kinase (PI3K) signaling pathway in TSH-stimulated thyroid cells reduces NIS expression at the transcriptional level. We, therefore, investigated the effects of PI3K pathway inhibition on iodide uptake and NIS expression in rat thyroid cell lines and human papillary thyroid cancer cells. A PI3K inhibitor, LY294002, significantly enhanced iodide uptake in two rat thyroid cell lines, FRTL-5 and PCCL3. The induction of Nis mRNA by LY294002 occurred 6 h after treatment, and was abolished by a translation inhibitor, cycloheximide. Expression of the transcription factor, Pax8, which stimulates NIS expression, was significantly increased in PCCL3 cells after LY294002 treatment. Removal of insulin abrogated the stimulatory effects of LY294002 on NIS mRNA and protein expression, but not on iodide uptake. These findings suggest that PI3K pathway inhibition results in post-translational stimulation of NIS. Inhibition of the PI3K pathway also significantly increased iodide uptake ( approximately 3.5-fold) in BHP 2-7 papillary thyroid cancer cells (Ret/PTC1 positive), engineered to constitutively express NIS. Pharmacological inhibition of Akt, a factor stimulated by the PI3K pathway, increased exogenous NIS expression in BHP 2-7 as was seen with LY294002, but not increase the endogenous NIS expression in FRTL-5 cells. PI3K pathway inhibition increases functional NIS expression in rat thyroid cells and some papillary thyroid cancer cells by several mechanisms. PI3K inhibitors have the potential to increase radioiodide accumulation in some differentiated thyroid cancer.