The change in thyroid function categories with time in patients with subclinical hypothyroidism: a systematic review and meta-analysis

Background Subclinical hypothyroidism (SCH) is characterized by elevated levels of thyroid hormone (TSH) and normal levels of free thyroxine (FT4). The outcomes of SCH patients are crucial for determining treatment plans; therefore, our aim is to summarize the existing prospective studies to understand the changes in thyroid function over time in SCH patients and the factors influencing these changes, providing references for clinical diagnosis and treatment. Methods We searched PubMed, Embase, Cochrane Library, and Web of Science for prospective follow-up studies on natural outcomes of SCH published until September 2024. Results are presented as the overall risk ratio (RR) and 95% confidence intervals (CI). Results We reviewed 8 prospective follow-up studies involving 1,859 individuals and extracted data from them for a meta-analysis. We found that when TSH levels are >= 10 mU/L, patients with SCH are more likely to progress to overt hypothyroidism (OH) (RR11.38, 95%CI 4.98-26.03, P<0.001) and were less likely to return to normal TSH levels (RR 0.20, 95%CI 0.09-0.42, P<0.001) compared to patients with TSH between 4.5 and 9.9 mU/L. In addition, patients who test positive for thyroid peroxidase antibodies (TPOAb) are more likely to progress to OH (RR 2.53, 95% CI 1.86-3.44, P < 0.001) and less likely to return to euthyroidism (RR 0.68, 95% CI 0.60-0.76, P < 0.001) compared to TPOAb-negative patients. Conclusion The results indicated that a large proportion of patients diagnosed with SCH will return to normal TSH levels or maintain SCH. Additionally, patients with TSH levels >= 10 mU/L or positive for TPOAb are more likely to experience progression and should be closely monitored. However, we did not find any gender differences in the natural outcome of SCH.