Understanding the clinical and molecular basis of thyroid orbitopathy: a review of recent evidence

The eye symptoms and signs that occur associated with thyroid dysfunction are called thyroid eye disease (TED), also known as Graves’ ophthalmopathy or orbitopathy (GO), thyroid-associated ophthalmopathy (TAO), and thyroid orbitopathy (TAO). According to the literature, 90% of patients with TED have hyperthyroidism, 5% are hypothyroid, and another 5% are euthyroid, thus confirming that thyroid levels are not an isolated risk factor [4]. TED occurs in only 25–50% of patients with AITD [5]. Tanda et al. reported that 73.7% of newly diagnosed thyroid patients had no ocular involvement, 20.2% had mild and inactive TED, 5.8% had moderate to severe and active TED, and 0.3% had compressive optic neuropathy in an Italian population [6].

Lid retraction as a sign of TED was described in the nineteenth century by Dalrymple working at the Royal London Ophthalmic Hospital. The sign referred to as lid lag (also known as Graefe’s sign, named a/er the German ophthalmologist Von Graefe). Gorman and Bartley’s diagnostic criteria for TED are as follows. If eyelid retraction occurs in association with any one of the following: thyroid dysfunction, exophthalmos, optic nerve dysfunction, or extraocular muscle involvement, then the patient is said to have TED. If lid retraction is absent, then TED may be diagnosed only if exophthalmos, optic nerve involvement, or restrictive extraocular myopathy is associated with thyroid dysfunction [7].

Frueh’s diagnostic criteria are more objective for the diagnosis of TED [8]. Here, at least two of the following clinical features need to be met: a history of thyroid dysfunction, exophthalmos >20 mm, lid retraction (>7mm), extraocular muscle involvement, bilaterality on CT scan, or bilaterality of the above clinical features was proposed as being diagnostic of thyroid eye disease.

The natural course of TED has been described as following Rundle’s curve, which has two phases, namely, an active and an inactive phase [5]. The initial active inflammatory phase is characterized by periorbital erythema and edema, orbital inflammation, conjunctival chemosis and congestion, and upper lid retraction. The inflammatory phase typically lasts for a period of between 6 and 24 months and is followed by a quiet, minimally inflammatory chronic fibrotic phase, which is associated with orbital fibrosis, glycosaminoglycan deposition, and enlarged extraocular muscles.

Douglas et al. developed a clinical response index for TED using the Delphi method [13]. Their goal was to develop an instrument that can be used for clinical assessment in prospective, longitudinal, and observational clinical trials. Activity is measured by various scoring systems, such as the clinical activity score of Mourits et al., NO SPECS (no physical signs, only signs, soft tissue involvement, proptosis, extra ocular muscle involvement, corneal involvement, and sight loss due to optic nerve compression), and VISA (vision, inflammation, strabismus, and appearance) scoring [14]. VISA scoring is considered to be practical as it can prognosticate during follow-up visits. Based on the VISA classification, the disease is classified as active or inactive, where a score above 4 out of 8 is considered active. Active disease needs immediate treatment to avoid potential complications, whereas inactive disease can be managed conservatively [15]. Severity is assessed by the European Group on Graves’ orbitopathy (EUGOGO) [16]. Based on these guidelines, the disease is classified as mild, moderate, or severe. Thyroid-stimulating immunoglobulin can be detected by bioassays which serve as prognostic indicators of the activity of the disease with 97% sensitivity and 90% specificity.

There is abundant literature evidence that TED affects quality of life (QOL). Terwee et al. developed a QOL questionnaire specific for TED with two subscales of eight questions each [17]. The first subscale was based on vision, including double vision, and the second on esthetic appearance. Park et al. modified the questionnaire by dividing it into three sections containing 19 questions [18]. According to several studies, TED dramatically decreases QOL. Delampady et al. validated TED QOL in the Hindi language in India and showed a reduction in QOL among those with TED [19]. They also added that in their population, there are various limitations in the visual function subscale in carrying out such activities as using public transport or working in agricultural fields, driving, and reading as most of their patients are from a rural background. Lee et al. [20] performed an elaborate analysis of existing questionnaires and reported that the TED questionnaire for QOL by Terwee et al. is faster and can be done in a busy outpatient department, whereas the Park et al. QOL questionnaire is elaborate and time-consuming. Stephanie et al. performed a meta-analysis to study QOL in TED [21].They concluded that the majority of them had a negative impact on QOL and psychosocial function. Thyroid-related Patient Reported Outcome questionnaire (ThyPRO) is yet another such questionnaire that evaluates thyroid patients’ QOL. The authors have recommended its use in clinical trials as it could additionally evaluate the relationships between clinical variables and QOL [22].

There are multiple mechanisms at work in TED, including autoimmune dysregulation, oxidative stress, hypermetabolic state, and vitamin D level imbalance. As mentioned earlier, the key underlying biology of this disease is the autoimmune-induced development of antibodies. Due to the dysregulation of the individual’s systemic thyroid status, there is infiltration of immune cells into the orbital tissue that activates the T and B cells, which produce antibodies to the TSH and the IGF-1 receptors present in the orbital fibroblast. Orbital fibroblasts appear to be the main target in TED, as they express both of these receptors. Antibody production against self-antigens (TSH R and IGF R) initiates the cascade of reactions that occur in TED. Autoantibody production is also noted in thyroid peroxidase (also known as antimicrosomal antibody) and thyroglobulin. Antibodies to the TSH receptor are also known as thyroid receptor antibodies (TRAb). TRAb is a heterogenous group of antibodies that can be thyroid-stimulating antibodies (TSAb), thyroid-blocking antibodies (TBAb), or neutral antibodies [23]. These immunoglobulins in turn activate the inflammatory cascade.

TSAb (TSH receptor-stimulating antibody) stimulates TSH-R, causing increased cyclic adenosine 3′,5′-monophosphate (cAMP), which increases the synthesis of thyroxine (T4) and triiodothyronine (T3). TBAb (a TSH receptor–blocking antibody) inhibits the function of the TSH-R and decreases the synthesis rate of thyroid hormones. Neutral Ab is able to induce variable signaling cascades, some of which are initiated by TSAb [24]. Both GD and HT are caused by lymphocytic infiltration in the thyroid parenchyma. In GD, since the infiltration is mild, the gland remains intact. In HD, the lymphocytic infiltrate causes the destruction of the follicles, which may lead to hypothyroidism [25].

The stimulation of these receptors by the antibodies can stimulate the inflammatory reaction, cause adipogenesis, and increase hyaluronan production [26]. TSH and IGF-1 receptors share common epitopes that are responsible for the physical and functional interactions in the fibroblast. Thus, inhibition of the IGF receptor can stop the progression of the cascading effects. Orbital fibroblasts in TED are different from normal orbital fibroblasts in terms of functionality and, hence, express abundant inflammatory cytokines [27].

The thyrotropin receptor (TSHR) was long thought to be the prime target in TED; however, recently, insulin-like growth factor-I receptor (IGF-IR) has been proposed as a second participating antigen in TED as it has physical and functional similarity, while inhibition of IGF-1R also causes signaling downstream from both receptors [28]. Teprotumumab, a human monoclonal antibody IGF-IR inhibitor, was used in moderate to severe active TED and was found to be safe and highly effective in reducing disease activity and severity. Induced cochlear deafness, however, was reported as a side effect. Targeting IGF-IR with specific biologic agents may result in a paradigm shift in the management of TED [29].

It has also been hypothesized that oxidative stress may cause the cascade of effects occurring in TED. Achieving a balance in the cell redox rate is recommended to maintain the normal homeostasis of cells. An increase in reactive oxygen species (ROS), such as hydroxyl radicals (OH^–), hydrogen peroxide (H₂O₂), superoxide anions (O2), and lipid peroxides, and a decrease in their clearance can cause oxidative stress and the breakdown of cellular homeostasis. Glutathione (GSH), superoxide dismutase (SOD), glutathione peroxidase (GPX), and catalase act as ROS antagonists and maintain cellular redox homeostasis [30].

Dettore et al. hypothesized that the autoimmune insult causes DNA methylation which elicits differential gene expression, the latter being responsible for mediating the pathogenesis of TED. The authors added that 58 genes were differentially expressed in TED compared with the controls [27]. Thyrotoxicosis is a hypermetabolic state characterized by high consumption of intracellular ATP and oxygen. An increase in ROS can cause damage to mitochondria and affect the production of oxygen and ATP. In vitro studies in rats showed that exposure to H₂O₂ caused selenium (Se) deficiency and DNA damage, resulting in mutations and triggering apoptosis and necrosis [31]. It is hypothesized that the increase in ROS can cause dual actions, as follows: (a) an increased release of autoantigens and production of TSH-R autoantibodies (TRAb), and (b) orbital tissue damage that contributes to the clinical manifestations of hyperthyroidism.

There are a few atypical cases which stained positive for IgG4 association between TED and IgG4-related orbital disease. Both the entities have an overlap in the underlying autoimmune fibrous inflammatory disease possibly due to Β cell depletion [32].

Genetic studies in TED have not been favored because of the small sample size of the previous studies and the inability to replicate the experiment. Moreover, the genetics of thyroid ophthalmopathy were mainly studied based on the genetic variants in GD and genes related to proinflammatory cytokines and immune regulation [33]. TED is a complex hereditary disorder which is, however, influenced by environmental factors; thus, it should be understood that the autoimmunity occurs when genetic susceptibility intersects with acquired factors. In the past two decades, common GD-associated genes identified and confirmed through linkage and association studies are HLA-DR1-Arg74 (human leukocytic antigen—DR beta 1 arginine at position 72), CTLA-4 (cytotoxic T lymphocytic antigen), PTPN22, CD40, CD25, TG (thyroglobulin), and the TSHR gene (thyroid-stimulating hormone receptor) [34].

Recently, Zou et al. used a data-driven approach to detect gene biomarkers in TED, which combines biomarkers both from those reported in the literature and those detected through their finding of differentially expressed genes in their study. Furthermore, a regulatory pathway of biomarkers was constructed, followed by various gene ontology-based enrichment analyses [35].

Epigenetic changes are modifications to DNA that cause genes to be turned on or off and thus influence the production of proteins in cells. They ensure that each cell produces only those proteins that are necessary for its function. These changes can be in the form of DNA methylation, histone modification, or non-coding RNA. In a Chinese study that compared the DNA methylation patterns between the healthy population and patients with TED, DNA methylation was significantly different between the groups in 148 genes. There are hyper- and hypomethylated genes, and the authors predict that they could be risk biomarkers for TED and could pave the way for new treatment strategies. In addition, decreased levels of ICAM1 methylation are significantly associated with exophthalmos in patients with GD [36].

Thyroid values are not the only risk factor, a large amount of literature having been published stating that TED is also influenced by deranged biochemical, hormonal, autoimmune, microbiome, and genetic factors. A multicenter cohort study concluded that free T3 and T4 are important predisposing risk factors for TED and that uncontrolled T3 and T4 play a critical role in the progression of the disease [37]. Female gender is a strong risk factor for TED; however, many studies report that men develop the most severe form of the disease. Middle-aged people are commonly affected, with the severity being greater in the elderly. White Caucasians are more commonly affected than Asians.

Smoking is an important risk factor for TED; however, it is not to date known which aggravates TED, smoke or cigare4e ingredients. The smoke (both active and passive) is believed to increase free radicals, causing oxidative damage that induces an inflammatory response leading to thyroid dysfunction [38]. According to the Tromso study, active current smoking is associated with a reduction in TSH levels in serum, which causes hyperthyroidism [39]. This was corroborated by The National Health And Nutrition Examination Survey (NHANES) study, which found significantly lower TSH values in active smoking patients (serum cotinine value of 15 ng/ml) compared to mild and occasional smokers [40]. The majority of studies have reported lower levels of TSH in patients with active smoking.

The effect of smoking on thyroid peroxidase is controversial and debatable. Thyroid peroxidase is an enzyme that adds iodine to thyroglobulin and is an important component of thyroid hormone synthesis [41]. Smoking has variable effects on thyroid function, which may be due to various constituents in tobacco, including carcinogens, alkaloids, and gases. Smoking has a protective effect on hypothyroidism and a negative influence on hyperthyroidism. Cigarette smoking is associated with increased orbital venous congestion in patients with TED [42].

Medications causing abnormal thyroid function are amiodarone, lithium, interferon, tyrosine kinase inhibitors, rifampin, and anti-epileptics. Tyrosine kinase inhibitors are targeted therapies approved for the treatment of several hematological tumors [56]. There are various hypotheses explaining thyroid dysfunction, including destructive thyroiditis and inhibition of vascular endothelial growth factor (causing alteration of thyroid function). There are few case reports of hypothyroidism induced by rifampin [57].

Amiodarone is an anti-arrhythmic agent that affects the function of the thyroid gland because of its rich iodine content. Patients treated with amiodarone may manifest clinically significant amiodarone-induced hypothyroidism or amiodarone-induced thyrotoxicosis. Amiodarone-induced hypothyroidism is due to the inability of the thyroid to escape the Wolf-Chaikoff effect, and it prevails in areas with high iodine content. Amiodarone-induced hyperthyroidism occurs more frequently in areas with low iodine intake due to iodine-induced excessive thyroid hormone synthesis (type I) or destructive thyroiditis with release of preformed hormones (type II) [58].

Surgical management is generally required for cosmetic correction or functionally when there is dysthyroid optic neuropathy. The disease can cause disfiguring proptosis, lid retraction, or squint. Ideally, the order of surgery should be orbit, lid, and then squint. Proptosis is the most visible and obvious side effect of TED, which requires earlier correction. Correction of orbital proptosis itself could improve lid and squint and can then be adjusted if revisions are needed [79]. Orbital decompression involves removal of one or multiple orbital walls with or without orbital fat removal to increase orbital volume and reduce the pressure exerted by the expanded extraocular muscles on the optic nerve. Indications for this surgery include compressive optic neuropathy, disfiguring proptosis, exposure keratopathy, globe subluxation, and/or uncontrollable elevation of intraocular pressure. The patient should be euthyroid when cosmetic corrections are undertaken.

Biomarkers can help in prediction of the disease. They can also be targets for treatment, such as the IGFR receptor antibody (teprotumumab), which is currently being used to treat active TED. With the advent of newer molecular target therapies, earlier treatment can be initiated for TED even before the activity develops. This could in turn avoid the complications with current treatment protocol.

As the biogenesis of the disease is an inflammatory cascade, proinflammatory cytokines and chemokines including IL-1, IL-6, IL-10, IL-8, C–C chemokine ligand 20 (CCL20), and IL-17 are elevated. Studies have shown that these markers can also indicate the stage of activity of the disease and report that the blood levels of some cytokines reflect the response to treatment: patients with refractory TED have higher levels of IL-4, IL-6, and IL-10 than patients in remission. Intercellular adhesion molecule-1 (ICAM-1) and soluble vascular cell adhesion molecule-1 (sVCAM-1) have been found to be elevated in the blood of TED patients as compared to controls [35].