Hypercytokinemia has been postulated as a pathogenesis of TS, but there are no previous reports in which there was measurement of cytokines in actual patients. The present study confirms that serum IL-6 is markedly elevated in patients with TS compared with in patients with Graves' disease.
In the animal experiments, we successfully induced pathophysiological changes similar to those observed in human TS by externally administering thyroid hormone and LPS to mice. To reproduce the pathogenesis of TS, in which thyrotoxicosis is combined with infection, we administered T3 on a daily basis beforehand (once per day for seven consecutive days), followed by concurrent LPS administration (once, on day 7). This approach not only significantly increased IL-6, metanephrines and ALT in blood biomarkers, it also increased MSS and mortality. TS may induce liver damage due to circulatory and/or metabolic disturbances in the liver [15]. Additionally, beta-blockers have been shown to be effective against cardiovascular symptoms of Graves' disease and TS, so sympathomimetic effects could be involved in these conditions [17]. No obvious changes in heart and liver pathology were observed, possibly because of the short time of 4 hours after LPS administration. The elevated levels of metanephrines and ALT observed in our model were thought to resemble the pathologies seen in human TS. The marked elevation of IL-6 in our model is also consistent with the aforementioned results observed in patients with TS, which could support the reproduction of hypercytokinemia. Our model is therefore considered to highly resemble human TS, characterized by liver injury, heightened sympathetic activation, and hypercytokinemia.
In this model, neither T3 nor LPS alone caused an increase in MSS or mortality, but their combination resulted in a significant increase. The synergistic effect of thyroid hormone excess and infection was consistent with the condition observed in human TS. In patients with thyrotoxicosis, macrophages, osteoblasts, and other cells are known to release IL-6 in response to T3 in patients with thyrotoxicosis [5, 6]. The release of IL-6 is therefore assumed to be more enhanced when thyrotoxicosis is complicated by infection. Furthermore, IL-6 itself has been recognized as an exacerbating factor in sepsis [18]. Hypercytokinemia can therefore cause multi-organ failure and death in TS.
The TS model in the present study was used to investigate whether ghrelin is useful in the treatment of TS. Ghrelin treatment was shown to significantly reduce IL-6 and metanephrine levels and to improve survival and MSS. The anti-inflammatory and sympathetic nerve activity suppressing effects of ghrelin are suggested by these findings to be highly beneficial for the treatment of TS [11]. ALT did not show significant changes within the initial 4 hours after ghrelin administration, but this lack of significant change can be attributed to the long half-life of approximately 25 hours [19] .
Ghrelin is known to bind to GHS-R1a expressed on monocytes and macrophages, directly inhibiting the production of IL-6 and IL-1β [20]. In addition, ghrelin binding to GHS-R1a expressed on the nerve endings of the vagal afferent tract activates the vagus nerve and indirectly suppresses NF-κB signaling in macrophages [21]. Ghrelin has been demonstrated to have positive effects on survival, the inhibition of inflammatory cytokine production, and the reduction of acute kidney and lung injuries in sepsis models [12, 22]. Clinical trials of ghrelin administration in humans have reported anti-inflammatory effects and improved exercise tolerance and food intake in patients with chronic respiratory failure [23]. Ghrelin has also been shown to reduce blood noradrenaline levels, to maintain left ventricular contractility, and to improve muscle strength and exercise tolerance in patients with chronic heart failure [24]. Moreover, the ghrelin receptor agonist anamorelin has been recently approved for use in Japan to manage cancer-related cachexia [25]. As clinical research progresses, indications for administration of ghrelin are expected to expand to include sepsis, chronic respiratory failure, and chronic heart failure. Our study suggests that in the future, ghrelin could be clinically applicable in the treatment of TS.
This study has some limitations. Firstly, due to the rarity of TS, it was challenging to collect a large sample of patients, and this resulted in a small sample of patients in this study that could be evaluated for serum IL-6 levels. However, the difference in serum IL-6 levels between patients with and without TS was statistically significant. Secondly, TS can be triggered by a variety of factors. Further investigation is required to determine whether our animal model is also applicable to TS caused by other triggers. Thirdly, although the dosage and timing of ghrelin administration used in the present study were based on the results and findings of previous studies [10], further studies might be required to find the optimal dosage and timing. Finally, there has been no elucidation of molecular mechanisms such as the interaction between ghrelin and cytokine signaling. Further challenging research is therefore necessary to clarify these questions.