Exploratory Analysis of Serum IGF-I Levels and Symptom Trajectories in Long COVID During the Omicron Period

Background: Although several risk factors have been reported for long COVID (LC), reliable biomarkers for this illness remain lacking. Insulin-like growth factor (IGF)-I, a major mediator of growth hormones, plays an important role in metabolism, neuroprotection, and systemic homeostasis, and therefore may be useful in predicting the severity and prognosis of LC. Methods: This study included patients who visited a specialized clinic for long COVID between 2022 and 2025 during the Omicron period and had serum IGF-I measurements taken. IGF-I levels were expressed as age- and sex-adjusted standard deviation scores (IGF-I SD), and patients were classified into low (SD < −1.0), normal (−1.0 ≤ SD < 1.0), and high (SD ≥ 1.0) groups. Clinical characteristics, patient-reported outcomes, laboratory data, and follow-up duration were analyzed. Results: A total of 811 patients were included (median 42 years; 52.5% female). Compared with the normal group, the low-IGF-I group exhibited higher fatigue (FAS: 37.0 vs. 34.0; p < 0.05) and depressive (SDS: 50.0 vs. 49.0; p < 0.05) status. Multivariable linear regression analyses identified lower IGF-I SD as independently associated with higher scores of both FAS and SDS. IGF-I SD values showed negative correlations with ferritin (ρ = −0.125, p < 0.05) and TSH (ρ = −0.202, p < 0.01) and positive correlations with albumin (ρ = 0.227, p < 0.01) and FT4 (ρ = 0.165, p < 0.01). Among the 237 patients who completed follow-up, the median duration from the initial visit to recovery tended to be longer in the low-IGF-I group (221 days) compared with the normal (191 days) and high (109 days) groups, although these differences were not statistically significant overall. In patients aged < 50 years, the low-IGF-I group showed a relatively longer follow-up duration (p < 0.05). Furthermore, the low-IGF-I group had a longer time to recovery compared to the high-IGF-I group (p < 0.05), and this difference was more pronounced in patients under 50 years of age, with significant differences observed among the three IGF-I groups. Conclusions: Lower IGF-I levels in LC may be associated with greater fatigue and depressive symptoms and longer recovery time, particularly in younger patients. Further studies are needed to clarify the clinical significance of these findings.