The novel coronavirus, which is the cause of the Corona Virus Disease 2019 (COVID-19), has had a significant impact on human life and health. COVID-19 enters host cells to initiate immunity through the binding of angiotensin-converting enzyme 2 (ACE2) or transmembrane protease, serine 2 (TMPRSS2). The high expression of ACE2 and TMPRSS2 in the thyroid gland is even higher than that in lung tissue, so the thyroid gland may be directly affected by COVID-19 (1).

Non-thyroidal illness syndrome (NTIs) are patients without previous basic thyroid diseases. Due to severe systemic diseases, thyroid hormone levels in the blood circulation are abnormally changed. This change is mainly manifested in the decrease of total triiodothyronine (TT₃) and free triiodothyronine (FT₃) levels, normal or decreased levels of total thyroxine (TT₄) and free thyroxine (FT₄), increased levels of reverse T₃ (rT₃), and thyroid stimulating hormone (TSH) levels are in the normal range (2).

It has been observed that many patients diagnosed with COVID-19 often experience abnormal changes in their thyroid hormones, which is known as NTIs. Present, a number of studies have found that FT₃ levels in patients with COVID-19 are significantly reduced. Zhang et al. found that 28% of COVID-19 patients had thyroid diseases, mainly NTIs (48%) (3), and Łukasz and his colleagues found that COVID-19 patients had a significant decrease in FT₃ (4). COVID-19 caused changes in thyroid hormones, which may be related to the down-regulation of 5’-deiodinase activity caused by cytokine storm, in vivo consumption affecting serum thyroid hormone transporter levels, hypothalamic-pituitary-thyroid axis (HPT) dysfunction and other factors (2). Six months after COVID-19 infection, many people still feel muscle pain, muscle weakness (mild to severe), fatigue and exercise intolerance. Among them, fatigue plays a major role in long COVID syndrome (5).

Currently, there is limited research on the relationship between thyroid hormones and post-COVID syndrome both domestically and internationally. The objective of this study is to investigate the connection between thyroid hormones and long COVID syndrome-fatigue. Additionally, the study aims to identify risk factors that contribute to persistent fatigue following COVID-19 infection. The findings of this research can serve as a guide for the diagnosis, treatment, and comprehensive management of COVID-19 sequelae.

COVID-19 can affect multiple systems throughout the body. As an important organ of the endocrine system, the thyroid gland is directly or indirectly affected by COVID-19. The effect of COVID-19 on the thyroid gland may be caused by directly (caused by direct cytotoxicity of the virus) or indirectly (caused by abnormal immune inflammatory response to the virus, which may involve coagulation, cytokines and complement systems) (9). NTIs is common in COVID-19 patients. A retrospective cohort study of Zou et al. included 149 COVID-19 patients, of which 41 (27.52%) were diagnosed with NTIs (10). A prospective study included 196 patients diagnosed with COVID-19 and found that 60% of ICU patients and 36% of general ward patients showed NTIs (11). In our study, 57.5% of all follow-up patients showed NTIs.

The main manifestation of NTIs is the decrease in T₃ level. The mechanism includes the change of half-life of thyroid hormone in circulation, the change of cell sensitivity to thyroid hormone, the change of tissue uptake of thyroid hormone and the change of deiodinase activity which converts thyroid hormone into active and inactive forms respectively (12). After T₄ deiodination, plasma T₃ is mainly derived from the role of DIO1 in the liver. During NTIs, the activity of DIO1 in the liver and DIO2 in skeletal muscle decreased, and the activity of DIO3 in the liver and skeletal muscle increased (13). This may be a physiological adaptation to reduce energy needs during acute diseases. Under high energy demand conditions such as diseases, skeletal muscle is the main organ responsible for glucose uptake in response to insulin. In this case, muscle protein catabolism can be stimulated to maintain energy in other organs, and a decrease in T₃ levels reduces metabolic activity and reduces energy loss in these patients (14–17).

After the recovery of COVID-19, some people still suffer from persistent and periodic symptoms. A Chinese study that included 1733 patients who had been hospitalized for COVID-19 found that at least one skeletal muscle-related symptom was still present in 76% of patients at 6 months, with fatigue or muscle weakness accounting for 63% of these (18). An Italian study reported fatigue as the predominant symptom (53.1%) of persistent symptoms 60 days after a COVID-19 infection (19). A meta-analysis showed that fatigue was the most common symptom of long covid syndrome (28.4%). Even in the non-hospitalized population, the most common long covid syndrome symptom is still fatigue, which is present in 34.8% (20). Therefore, chronic fatigue syndrome (CFS) is the most common complaint of patients. CFS, also known as myalgia encephalomyelitis, is a complex heterogeneous disorder characterized by disabling fatigue, cognitive impairment, sleep disruption, and accompanying bone and muscle pain that persists for more than 6 months and does not improve with rest (21, 22). The global pooled prevalence of CFS among long covid syndrome patients is 45.2% (23). A meta-analysis showed that the total prevalence reported by Western and Asian populations was comparable (1.32 ± 1.45% vs 1.51 ± 1.74) (24). A prospective study showed that 75.9% of patients reported fatigue 3-6 months after infection with COVID-19, which was consistent with our study (5). In our study, 70% of patients were still fatigued 6 months after infection with COVID-19. The prevalence of CFS has been reported to be 2-3 times higher in females than in males, but there was no significant gender difference in this study (25). A retrospective study showed that 48.9% of CFS patients diagnosed after COVID-19 infection were male and 51.1% were female (26). We speculate that this may be due to the fact that infections are a significant trigger for the development of CFS, and that infections make the gender difference negligible.

CFS may be associated with changes in hormone levels in the hypothalamus-pituitary-thyroid axis (27). Studies have shown that thyroid hormone function in CFS is similar to NTIs. A Serbian study found that compared with the healthy group, although the basal serum T₃ of all subjects was normal, the T₃ concentration in the CFS group was significantly reduced (28). In this study, the level of FT₃ in the fatigue group was significantly lower than that in the non-fatigue group (2.52 ± 0.63 vs 3.00 ± 0.56 p< 0.05), and 64.3% of the fatigue group showed NTIs. FT₃/FT₄ is commonly used to evaluate the sensitivity of peripheral thyroid, which reflects 5’-deiodinase activity. SPINA-GD represents the total activity of peripheral deiodinase. This study shows that although there is no statistical difference in SPINA-GD between the two groups, SPINA-GD in the fatigue group is lower than that in the non-fatigue group. Correlation analysis shows that FT₃/FT₄ and SPINA-GD are related to fatigue (p<0.05), which indicates that fatigue in patients with COVID-19 may be related to subtle changes in deiodinase. Although NTIs may be beneficial in the acute phase of critical illness, it may hinder the recovery of patients in the case of long-term critical illness (29). Low T₃ may affect brain tissue perfusion and energy metabolism, leading to fatigue (30). This study shows that low T₃ is a risk factor for fatigue. When FT₃ value is less than 2.47 pmol/L, it is the best critical value to predict fatigue in COVID-19 patient.

Even though the study has provided interesting findings, it had some few limitations. Certain limitations of the study are as follows: Firstly, the sample size is relatively small, resulting in a higher number of patients lost to follow-up, which may introduce a bias of loss of follow-up. Secondly. The patients included were all Chinese, potentially limiting the generalizability of the findings to other regions or ethnic groups. Thirdly, the thyroid hormone levels of COVID-19 patients after 6 months were not measured again, potentially limiting the understanding of the long-term effects of thyroid hormones on CFS. Fourthly, the average age of both groups is over 60 years old, and there is a lack of data on the young population. Finally, a meta study showed that the prevalence of CFS was 0.10% based on the physician diagnosis, and the prevalence of CFS was 2.03% in the questionnaire interview without medical testing (24). Therefore, the prevalence of CFS based on the questionnaire survey in our study may be overestimated. In the future, large-scale prospective studies are needed to further explore the mechanism of thyroid hormones affecting CFS after COVID-19.

Furthermore, in addition to thyroid hormones, symptoms of chronic fatigue may also be caused by damage to multiple organ systems during COVID-19. The occurrence of CFS in COVID-19 may be related to excessive anti-inflammatory response, and the dominant disease pattern of alternatively polarized macrophage cells directly induces the occurrence of CFS (31). Hypothalamic-pituitary-adrenal (HPA) axis disturbances have also been found in CFS, which are manifested by mild cortisol decrease, enhanced negative feedback, and a blunted response to excitation (32). COVID-19 can also cause skeletal muscle damage resulting from changes in the structure and function of the neuromuscular system through mechanisms such as direct infiltration, inflammatory response, hypoxia and other mechanisms to cause skeletal muscle damage, resulting in fatigue (33). A study reported that the immune system of ME/CFS patients produces an imbalanced ratio of pro-inflammatory and anti-inflammatory cytokines in the early stages of the disease (34). But CRP and PCT are not statistically significant in our study. In conclusion, the occurrence of CFS in COVID-19 patients is not a role of a single hormonal axis, but a combination of factors.

In summary, this study demonstrates that the majority of patients still experience fatigue six months after contracting the COVID-19 infection. The level of FT₃ in the fatigue group was lower, suggesting a potential correlation between FT₃ and the persistence of fatigue following COVID-19 infection. The mechanism of fatigue may be related to subtle changes in the hypothalamus-pituitary-thyroid axis and deiodinase. In addition, this study found that low T₃ is a risk factor for fatigue. When the value of FT₃ is less than 2.47 pmol/L, it serves as the optimal critical threshold to predict fatigue in patients with COVID-19. In the future, prospective studies are needed to confirm whether monitoring serum FT₃ level can predict the occurrence of CFS in patients with COVID-19, and then help to improve the symptoms of patients.