Bexarotene (Targretin), approved since 1999 as a second-line treatment for late stage cutaneous T-cell lymphomas, has been shown to induce significant hypothyroidism through TSH suppression. This review revisits, through a case report, mechanisms by which rexinoids repress the expression of TSHB gene as well as αTSH and TRH genes. It appears that rexinoids suppress TSH independently from tri-iodothyronine. Bexarotene also differently affects the gene expression of deiodinases 1 and 2 as well as the peripheral clearance of thyroxine. These data might open new ways of research on the potential interaction between thyroid axis and endogenous rexinoids.
Invited Author's profile
Jacques Orgiazzi is professor of Endocrinology, Claude-Bernard Lyon 1 University, and past head of the Service of Endocrinology, Centre Hospitalier Lyon-Sud, University hospital of the Hospices Civils de Lyon. His research, clinical and basic, has been mainly devoted to thyroid autoimmunity, more specifically, to the biology and clinical implication of the anti-TSH receptor antibodies, including the blocking variant identified in the course of these studies. More recently, he worked with the European Consortium ‘EUGOGO’ to improve knowledge and management of Thyroid Associated Orbitopathy.
In 2002, C Asteria published in the European Journal of Endocrinology a highlight paper entitled: ‘Treatment with retinoid X receptor-selective ligand (bexarotene) may cause iatrogenic central hypothyroidism’ which described a new mechanism of thyroid-stimulating hormone (TSH) suppression (1). More than 10 years later, taking advantage of the oncologists' and dermatologists' experience with bexarotene therapy and of the better understanding of the mechanisms of action of thyroid hormone and other nuclear receptor ligands, it is appropriate and timely to revisit the question.
Mycosis fungoides was diagnosed in a 67-year-old woman in September 2011. Since 2005, she had presented with prurigo and hypereosinophilia, initially treated with dermocorticoids and psoralen ultraviolet A (PUVA). In October 2011, she received methotrexate which had to be stopped because of clinical inefficiency and side effects. On November 7, 2011, bexarotene treatment was initiated. Initial daily dose of bexarotene was 300 mg and increased to 450 mg 1 month later. The patient complained of asthenia and of cold intolerance but her weight (52 kg; BMI: 21.6 kg/m2) did not change. On January 9, 2012, at routine thyroid biological workup, serum free thyroxine (FT4) was 3.5 pmol/l (normal range: 13.0–22.6), free tri-iodothyronine (FT3) 1.6 pmol/l (normal range: 2.8–5.3) and TSH 0.86 mU/l (normal range: 0.29–3.80). Morning plasma cortisol and adrenocorticotropin were normal as well as the post-menopausal serum gonadotropin levels (luteinizing hormone, 10.1 UI/l (8.0–33.0); follicle-stimulating hormone, 37.9 UI/l (23.0–116); prolactin, 11.0 ng/ml (1.8–20.3); insulin-like growth factor 1, 120.2 μg/l (54.0–204.4); and growth hormone, 2.5 mU/l (<20)).
Before initiation of bexarotene treatment, thyroid tests were normal (TSH: 3.3 mU/l; FT4: 13.0 pmol/l, FT3: 3 pmol/l). Levothyroxine (l-T4) was started at a daily dose of 50 μg and then progressively increased. On l-T4, serum TSH remained undetectable throughout the bexarotene treatment period while FT4 values remained infra-normal (7.0–11.0 pmol/l) until the l-T4 dose was finally increased to 200 μg/day (4 μg/kg per day) with serum FT4 reaching 16.7 pmol/l. Atorvastatin, 10 mg/day, had been prescribed along with bexarotene. However, because of disease progression, bexarotene was stopped in May 2013 and replaced by gemcitabine. l-T4 was then withdrawn. Three weeks later, serum TSH was normal (3.08 mU/l) but FT4 was low (9.4 pmol/l), FT3 being high normal (5.0 pmol/l). Four months after the end of the bexarotene treatment TSH was normal at 2.4 mU/l; FT4, 11.5 pmol/l (normal range: 9–19); and FT3, 5.8 pmol/l (normal range: 2.6–5.8) (Fig. 1).
Cutaneous T cell lymphoma
The gastrointestinal tract and the skin are the tissues more frequently affected with non-Hodgkin lymphoma. Cutaneous T-cell lymphomas represent ∼80% of all primary cutaneous lymphomas. According to the WHO-EORTC classification, mycosis fungoides, an indolent proliferation of epidermotropic T cells, is the more frequent form (50%) of the disease while Sezary syndrome, the aggressive erythrodermic leukemic variant, is uncommon (2). Clinically, mycosis fungoides occurs mainly in adults, with a median age of 55–60 years at diagnosis and a male-to-female ratio of about 1.8. Mycosis fungoides is restricted to the skin, with slow progression over the years to infiltrated plaques, ulcerations, and cutaneous tumors. The TNM classification differentiates T1–3 (skin lesions: eczematous patches, localized or generalized plaques, cutaneous tumors), N1–3, and M0–1 stages (3). Life expectancy is unaffected in T1–2 patients. The tumor–node–metastasis–blood-based staging criteria were revised in 2007 (4). The treatment algorithm of cutaneous T-cell lymphomas is based on the stage of the disease (4).
Retinoids, a group of structural and functional derivatives of vitamin A (retinol), are not only involved in vision as part of the rhodopsin molecule (the 11-cis-retinal) but also regulate complex gene networks involved in cell differentiation, proliferation, and apoptosis. Retinol is converted by alcohol dehydrogenase, then retinaldehyde dehydrogenase, to all-trans-retinoic acid, further isomerized to 13-cis-retinoic acid (isotretinoin) and 9-cis-retinoic acid in liver microsomes, depending on the levels of converting enzymes and cellular retinol-binding protein (5). The biological activity of all-trans- and 9-cis-retinoic acid is mediated by two families of nuclear receptors that act as ligand-dependent transcriptional regulators: the retinoic acid receptors (RARA, -B, and -γ) and the retinoid X receptors (RXRA, -B, and -γ) which form either homodimers or heterodimers with numerous nuclear hormone receptors (vitamin D receptor, peroxisome proliferator-activated receptors (PPAR), the orphan liver X receptor (LXR), farnesoid X receptor, constitutive androstane receptor, and thyroid hormone receptor (TR) (6, 7). Although RARs bind, and are activated by, both 9-cis- and all-trans-retinoic acids, RXRs are exclusively activated by natural 9-cis-retinoic acid, unsaturated fatty acids such as docosahexaenoic, linoleic, linolenic, and arachadonic acid and other natural ligands or synthetic ligands called rexinoids (8). Recently, a number of synthetic rexinoids have been developed, among which LG100268 and LGD1069 (bexarotene) are highly RXR selective (9).
Bexarotene: actions and side effects
Bexarotene (Targretin) has been approved since 1999 by the US Food and Drug Administration as a second-line treatment for early- and late-stage refractory cutaneous T-cell lymphomas and the European Medicines Agency for the treatment of advanced stages (IIB–IVB) of the disease in Europe. It is the only rexinoid approved for clinical use. Bexarotene is prescribed orally. The starting dose is 150 mg/m2 per day, to be adapted to clinical efficacy and tolerance. There is no evidence for any accumulation of the drug in the organism. It has a plasma peak 2 h after ingestion and a plasma half-life of 5–7 h (10, 11).
The mechanism of action of bexarotene on malignant T-cells is not completely understood, but the drug is thought to induce apoptosis through activation of the caspase-3 pathway and the poly(ADP-ribose) polymerase cleavage of (12) and/or inhibition of proliferation through the activation of the p53/73-dependent cell cycle pathway (13). The efficiency of bexarotene was also correlated with the inhibition of the mitogen-induced IL4 production by the peripheral blood cells (14). Bexarotene and other rexinoids are currently being studied as therapies for advanced lung, breast, and thyroid cancers (15) with, in the latter, restoration of RARB and RXRγ tissue expression and downregulation of NF-κB target genes (16). Rexinoids have many other effects. Among these, they exert beneficial glucose-lowering and insulin-sensitizing effects as well as antiobesity actions in animal models of insulin-resistance and diabetes, and bexarotene improves cholesterol homeostasis and inhibits the development of atherosclerosis in a mouse model of mixed dyslipidemia (17, 18). They also increase both hepatic liver oxidative metabolism through the increase in cytochrome P450, particularly the isoenzymes CYP4A, CYP2B1/2, and CYP3A levels, and the glycuronyltransferase activity (19). Finally, since bexarotene induces the transcription of apolipoprotein E and crosses the blood–brain barrier, it has been tested in a mouse model of Alzheimer's disease as a potential therapeutic agent to stimulate apolipoprotein E-induced clearance of β-amyloid with controversial conclusions (20).
The various side effects of bexarotene are listed Table 1. Some of them are nearly constant and appear to be dose dependent, likely related to the RXR agonist activity of the molecule. One of the initial phase II–III studies of the drug showed that 99% of the treated patients had experienced at least one significant adverse event (10). Central hypothyroidism (see the case report and below) and hypertriglyceridemia are the two expected side effects of bexarotene. Hypertriglyceridemia occurs with a median 5-week time interval after initiation of the treatment (21) in 82% of patients (10). It is severe (grade III and IV) in 11% of the patients, with pre-existing dyslipidemia being a risk factor (21). Hypertriglyceridemia (>8.5 mmol/l)-related pancreatitis is uncommon. Hypertriglycereridemia remains the more troublesome side effect despite combined lipid-lowering medications sometimes leading, in some patients, to bexarotene withdrawal. Triglyceride levels return to normal after treatment discontinuation. Elevated levels of LDL-cholesterol and low levels of HDL-cholesterol may also be observed in 30% of patients (10). Proper bexarotene treatment management and careful side-effects monitoring with routine laboratory tests minimize notably treatment drawbacks (22). As bexarotene is metabolized by cytochrome P450 3A4 (CYP3A4), interactions may occur with drug which affect CYP induction or activity (11). While the uncommon risk of hyperglycemia is unexplained, occurrence of severe hypoglycemia in insulin-treated patients is related to the bexarotene insulin sensitization effect (23). Table 1 lists the less common side effects of the drug.
Main bexarotene side effects. Table modified from (22).
|Side effect||Prevalence (%)||Comments|
|Central hypothyroidism||29–100||Dose dependent, requires preventive levothyroxine substitution|
|Hypertriglyceridemia||82–100||Dose dependent, median interval from treatment initiation: 5 weeks, risk of acute pancreatitis (triglycerides >8.5 mmol/l), requires preventive/curative lipid-lowering treatments|
|Increase in LDL-cholesterol||30||Usually easily manageable with lipid-lowering drugs|
|Decrease in HDL-cholesterol|
|Liver toxicity, elevated liver enzymes||11–20||Dose dependent, bexarotene discontinuation if elevation of transaminases greater than fivefold|
|Drug interactions with inhibitors or activators of the cytochrome P450 enzymes and GT||–||Dependent on drugs combination|
|Skin eruption, pruritus, mild phototoxicity||8–13||At treatment initiation|
|Leucopenia (neutropenia)||6–11||Late occurrence, especially in combination with chemotherapy|
|Uncommon||<5||Bexarotene enhances insulin action|
|Hypoglycemia in insulin-treated patients|
|Teratogenicity||–||Bexarotene contraindicated in women of childbearing age without contraception|
Bexarotene and the thyroid axis
It has been known for many years, in the context of vitamin A deficiency and/or supplementation trials, that vitamin A affects thyroid economy in several ways (24). Since 1995, a clearer picture has emerged, thanks to studies of the effects of available natural or synthetic vitamin A derivatives, including rexinoids, with narrow target specificity allowing for more specific action analysis. In 1978, Morley et al. (25) observed that vitamin A-deficient rats had significantly higher serum T4 and T3 levels than controls and that their pituitary TSH content was also increased suggesting a pattern of central hyperthyroidism. Breen et al. (26) observed that vitamin A deficiency led, in normal but not in hypothyroid rats, to a twofold increase in pituitary TSHβ mRNA levels which returned to normal 18 h after treatment with retinoic acid. Interestingly, there was no synergistic effect of the co-administration of retinoic acid and T3 on the TSHβ mRNA decrease in vitamin A-deficient animals (26). In 1997, Coya et al. (27) found that rats, either euthyroid or hypothyroid, treated with retinoic acid, showed a decrease in spontaneous basal TSH levels and TSH responses to TRH. In a quite different setting, data with similar meaning were recently reported in mildly iodine-deficient children in whom vitamin A (retinyl palmitate) supplementation decreased excess TSH stimulation thereby reducing the risk of goiter (28).
Bexarotene and the inhibition of TSH secretion
The occurrence of symptomatic central hypothyroidism in a patient with cutaneous T-cell lymphoma treated with bexarotene led Sherman et al. (29) to study the thyroid function of 27 patients with the same disease participating in the open-label study of high-dose (>300 mg/m2 per day) oral bexarotene. Severe central hypothyroidism occurred in 70% of these patients. Symptoms were usually poor (fatigability, cold intolerance, impaired cognition, constipation). Mean serum TSH declined from 2.2 to 0.05 mU/l, and mean FT4 from 12.9 to 5.8 pmol/l during bexarotene treatment. Correlation between nadir-to-baseline TSH ratio and the dose of bexarotene suggested a dose–effect relationship. After bexarotene discontinuation, among the 10/11 patients with normal thyroid function before bexarotene treatment, serum TSH concentrations returned to normal in nine as early as 8 days (29). In 1999, Dabon-Almirante et al. (30) reported on a woman with advanced cervical cancer treated with 9-cis-retinoic acid in whom TSH was suppressed and normalized spontaneously after drug withdrawal. Subsequently, Golden et al. (31) carried out a randomized, double-blind, crossover trial in six normal subjects who received a single dose of bexarotene, 400 mg/m2 of body surface area, or placebo. Serum TSH level had decreased as early as 12 h after drug administration, the nadir being reached at 24 h (0.32±0.02 vs 1.48±0.19 mU/l), and remained lower than in controls at 48 h (0.47±0.06 vs 1.80±0.2 mU/l). FT4 and FT3 indexes remained lower than in controls for 48 h. Serum prolactin and cortisol were not affected, as well as fasting and post-prandial blood glucose, triglyceride, insulin and free fatty acid levels (31), in accordance with our case report. To our knowledge, bexarotene does not affect the levels of the other pituitary hormones. Indeed, UK consensus statement on safe clinical prescription of bexarotene does not recommend exhaustive pituitary testing (11).
Bexarotene and the peripheral metabolism of thyroid hormones
Effects on thyroid hormone biodisponibility have also been suggested to explain the relative hypothyroidism observed in patients treated with bexarotene. Smit et al. studied ten athyreotic thyroid cancer patients with iodine uptake-negative pulmonary metastases who were on a 6-week redifferentiation treatment protocol with 300 mg/day bexarotene. During the bexarotene treatment protocol, the dose of l-T4 treatment was unchanged. Bexarotene and l-T4 were taken 12 h apart to prevent absorption interference. A marked decrease in serum total T4 (56% of baseline value) without alteration in serum TBG, as well as in serum FT4 (47%), total T3 (69%), and rT3 (51%), was observed in all patients, suggesting a bexarotene-induced increase in thyroid hormone metabolism. Serum TSH remained unchanged despite the decrease in thyroid hormone serum concentrations. The parallel decrease in T4, T3, and rT3 together with the modest increase in T3:rT3 ratio suggested to the authors that bexarotene did not significantly affect the three deiodinase activities (D1, D2, and D3). In contrast, serum T4 sulfate decreased to 70% of basal value while the T4 sulfate:FT4 ratio increased, which suggests an induction effect of bexarotene on the T4 sulfation and, possibly, glucuronidation pathways (32). These data are in line with the known activation by rexinoids of the liver metabolic oxidative enzymes as well as glucuronidation resulting in an increase of thyroid hormone metabolic clearance. However, at variance with these observations, rexinoid-induced modifications in deiodinase levels have indeed been reported in cell and animal models. In mice treated for 3 days with LG268, liver D1 mRNA and activity were found to be increased (5). In the pituitary, D2 mRNA was decreased but D2 activity remained unchanged. Interpretation of the pituitary data has to combine the direct negative effect of LG268 on the expression of the D2 gene and the indirect post-translational positive one on the D2 protein activity, the intracellular degradation of which is slowed by the T4 decrease resulting from the LG268 suppression of TSH. In the brain, D2 mRNA was unchanged while D2 activity was increased, likely also as a consequence of the T4 lower levels. Rexinoid suppression of the pituitary D2 gene expression, confirmed on the thyrotrope-derived pituitary cell line TAT1, appears tissue specific because it is not observed in the brain. In addition, rexinoid effect on D2 gene expression is RXR-selective as it is not observed with a selective RAR agonist or in mice with invalidated RXRγ (5).
As a whole, bexarotene-induced increase in peripheral clearance of thyroid hormone contributes to the drug-induced hypothyroidism. It also could explain the requirement for l-T4 doses higher in the treatment of bexarotene-induced hypothyroidism than hypothyroidism related to other causes. Figure 2 depicts the various levels of action of bexarotene on the thyroid economy.
Molecular aspects of the bexarotene-induced central hypothyroidism
RXRγ is the isoform implicated in bexarotene-induced central hypothyroidism
13-cis-retinoic acid, isotretinoid, a RAR agonist, has no effect on serum TSH levels, which confirms that the effects of retinoids on TSH are not mediated through RAR (33). In contrast, 9-cis-retinoic acid could suppress serum TSH in human (30). Expression of the three RXR isoforms is unevenly distributed among tissues. While RXRA and RXRB are expressed ubiquitously including in the anterior pituitary, RXRγ expression is strongly restricted to skeletal muscle, heart, brain, thyroid, and pituitary (34, 35), the RXRγ1 isoform being specifically expressed in the pituitary gland, especially in the thyrotrope cells, in mouse as well as man (36, 37, 38). Mice with inactivation of the Rxrγ gene disclose elevated levels of T4 and TSH and are relatively resistant to exogenous T3, a phenotype consistent with thyroid hormone resistance, suggesting the specific involvement of the RXRγ isoform in the regulation of pituitary TSH production (39). However, Tshb mRNA suppression by rexinoids in this model of mice lacking Rxrγ is not an all-or-none phenomenon. RXR is required at low dose of rexinoids, but other receptor isotypes can mediate the suppressive effect at high doses (5). On the same line of evidence, a selective RXR antagonist, LG101208, in vitro induces a 71–81%, at 24 and 48 h, increase in Tshb mRNA levels in TAT1 mouse thyrotrope cells, as well as an increase (53–47%) in the common glycoprotein α-subunit and D2 mRNA levels (40). In mice treated with LG101208, serum total T4 levels and T4:TSH ratio were significantly higher, serum TSH levels being slightly, but not significantly, higher than in control mice, a pattern compatible with a new thyroid ‘set point’ configuration. Pituitary Tshb and A-subunit mRNA levels were higher, but pituitary D2 levels were not different between treated and untreated animals (40). Altogether, these results highlight the potential role of RXRγ in the control of pituitary TSH production.
Bexarotene and the suppression of the TSHβ gene promoter activity
Several studies, both in vivo and in vitro, have demonstrated the suppressive effect of both all-trans- and 9-cis-retinoic acid on the specific TSHβ gene promoter (5, 9, 26, 37). Since the transactivation mediated by the binding of T3 to TR is facilitated by the formation of TR/RXR heterodimers, the first question arising had been whether bexarotene suppresses TSHB expression through binding to TR/RXR heterodimers (Fig. 2). The question is all the more appropriate as the mechanisms of the negative gene regulation by T3 are less well understood than those involved in positive regulation. At least three different models for the negative regulation by T3 have been proposed (41). For the regulation of TSHB expression, it is proposed that the unliganded TR bound minimally to the DNA of the TSHB negative thyroid hormone response element (nTRE) shifts to a strong binding in the presence of T3, with the recruitment of coregulators, possibly under TR conformation allosteric changes induced by the nTRE upon TR–T3 binding (42, 43, 44, 45).
Two lines of experimental data strongly suggest that bexarotene and similar rexinoids induce central hypothyroidism by direct binding of liganded RXR to specific response elements independently from any interaction with TRE:
retinoids suppress TSHB promoter activity through the −200 to −149 bp region of the mouse and rat promoter (37, 46), which is in sharp contrast to the location close to the transcription start sites of the TREs which mediate the negative effects of T3 (37, 46, 47) and
in mice deficient in the TRB isoform (TRB−/−) of the TR, treatment with a RXR agonist induced a potent and rapid suppression of serum TSH and T4 (48).
It should be reminded, however, that the molecular mechanisms involved in both the TR and RXR-negative regulations of the TSHB gene are still unclear, notably in what concerns their mutual interactions and the implication of the various endogenous coregulators within thyrotrope cells (49, 50). Similarly, it is still not clear whether the regulation of the TSHB promoter activity by rexinoids and T3 is additive or synergistic.
In addition to the above-described effect, a likely non-genomic rexinoid action on TSH secretion has been described by Liu et al. (9). In rats, a fall in serum TSH was apparent as early as 30 min after LG268 administration, with the nadir at 4–8 h, contrasting with the absence of reduction in pituitary TSHB mRNA and TSH protein at 2 h after LG268 administration. In the same experiment, TRH-stimulated TSH secretion was blunted which, together with the decrease in serum TSH, would suggest a rexinoid-induced alteration in the secretion/release process of the hormone, the mechanism of which is unknown. It is suggested that the time interval between rexinoid administration and pituitary TSHB mRNA and TSH content determination was too short for a change to be observed (9).
Does bexarotene influence the glycoprotein α-subunit and TRH gene expressions?
In TAT1 cell line, the rexinoid LG268 decreased mRNA levels of the glycoprotein α-subunit along with those of TSHB and D2 as mentioned earlier. Accordingly, pituitary α subunit mRNA levels, as those of TSHB, were higher in mice treated with the RXR antagonist (40), indicating that the α subunit and TSHB promoters share, at least partially, the same regulation.
Thyroid hormone control is also exerted on hypothalamic TRH production. nTREs have been identified in the human TRH gene promoter as three separate half-sites which act in combination for full promoter repression (51). Although two of these binding sites only bind TR monomers, the third one is able to bind TR monomers and homodimers or TR/RXR heterodimers (51). RXRs' influence on T3-mediated repression of TRH gene has been suggested by in vitro studies (51, 52, 53). In vivo administration of LG268 did not change the hypothalamus levels of TRH mRNA despite the concomitant decrease in serum TSH and T4 which, nevertheless, suggests some degree of suppression (5). Accordingly, in animals treated with the RXR antagonist, the levels of hypothalamic preproTRH mRNA levels were not different from controls despite increased T4 levels also indicating that the upregulation of the TRH gene expression by rexinoids cannot be excluded (40).
Altogether, it has been demonstrated that bexarotene is responsible for central hypothyroidism essentially through direct binding on specific regulatory regions of the TSHB gene, with coordinated effects on the α subunit and TRH production. It is unlikely that other bexarotene side effects, such as dyslipidemia, would result from direct alteration of the thyroid economy in the periphery. Rather, bexarotene-induced dyslipidemia is related to the permissive effect of RXR–LXR (18) and PPARα/RXR heterodimers (54, 55). Indeed, bexarotene has been shown to activate hepatic lipogenic genes such as SREBP1c, FAS, FAE, and SCD1, leading to increased triglycerides production via the RXR–LXR heterodimer in a dose-dependent manner (18). Recently, polymorphisms associated with bexarotene-induced high-grade hypertriglyceridemia were identified with potential in predicting bexarotene-improved survival response (56).
Recommendations for bexarotene treatment management
Before initiating bexarotene therapy, patients with a past history of hyperlipidaemia, diabetes or cardiovascular disease, and abnormal thyroid function should be identified. Concomitant medications affecting liver oxidative enzymes should be monitored appropriately. Full blood count, urea and electrolytes, liver and thyroid function tests, serum lipids, glucose, and creatinine kinase should be assessed (57). According to the recent UK recommendations, occurrence or worsening of hypertriglyceridemia should be prevented with fenofibrate or, if contraindicated, a statin treatment initiated 1 week before bexarotene treatment. Blood lipids should be tested on a weekly basis until control is obtained (11).
l-T4 may be started at the initiation of bexarotene (25–50 μg/day, with caution in patients with cardiovascular disease) or as soon as hypothyroidism is detected (50–100 μg/day). Serum FT4 (and T3), not TSH, should be measured monthly until doses of bexarotene and l-T4 have been stabilized. l-T4 treatment should be adapted in order to maintain FT4 level at the pre-bexarotene value. As it is obvious in the case report, the dose of l-T4 can be as high as 200–250 μg/day (58). Because of the usual prolonged effect of the induction of liver oxidative enzymes, discontinuation of the l-T4 treatment should be progressive after bexarotene discontinuation, guided on serum FT4 controls, as illustrated in the case report.
Bexarotene, through its nuclear receptor RXR, is able to regulate negatively the expression of the TSHβ gene and, at a lesser degree, of αTSH and TRH genes leading to central hypothyroidism, an effect observed in nearly 100% of the patients treated with 150 mg/m2 per day, warranting routine concomitant l-T4 treatment. Bexarotene also affects the expression of deiodinase D1 and D2 genes positively or negatively in the liver and pituitary respectively. In addition, bexarotene increases the metabolic clearance of T4. The central suppression of TSH appears independent from that of T3 thus providing a still theoretical way to directly interfere with the secretion of TSH in specific situations of resistance to thyroid hormones or thyroid cancer. The pharmacological rexinoid-mediated TSH alteration could also suggest a physiological role of endogenous RXR ligands, such as the polyunsaturated fatty acids, in the modulation of the thyroid axis set-point, well in line with current studies on the metabolism–thyroid axis interactions.
Declaration of interest
J Orgiazzi, V Vlaeminck-Guillem, J Graeppi-Dulac, and M Perier-Muzet have no conflict of interest that could be perceived as prejudicing the impartiality of the research reported to declare.
This review did not receive any specific grant from any funding agency in the public, commercial, or not-for-profit sector.
Author contribution statement
Each co-author contributed equally to the review.
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