Understanding Thyroid Hormone Dynamics After COVID-19 Infection

As SARS-CoV-2 continues to circulate globally, attention has shifted to its downstream effects in patients with chronic conditions. Thyroid disease has remained an area of interest, given the thyroid’s sensitivity to systemic illness and immune perturbation.

A retrospective study from a large thyroid disease center in Shanghai offers a focused look at how COVID-19 infection influences thyroid function and selected clinical indices in hypothyroid outpatients, with particular attention to differences between Hashimoto’s thyroiditis (HT) and non-HT etiologies.

Study Design and Patient Population
The investigators conducted a retrospective review of electronic health records from hypothyroid outpatients treated at a single tertiary thyroid center in Shanghai during the Omicron surge, spanning November 2022 through April 2023. Patients were excluded if they were younger than 18 years of age, had autoimmune diseases other than Hashimoto’s thyroiditis, were taking medications known to interfere with thyroid function, had recent adjustments to levothyroxine therapy, or had incomplete clinical data.

After these exclusions, 128 patients were eligible for analysis and were stratified into a Hashimoto’s thyroiditis (HT) group (n = 65) and a non-HT group (n = 63).

Thyroid function tests and related laboratory measures were evaluated at three standardized time points: prior to COVID-19 infection, approximately 30 days after infection (D30), and approximately 90 days after infection (D90), with all blood samples collected in the fasting state and before daily levothyroxine administration. Nearly 3/4 of the cohort had received an inactivated SARS-CoV-2 vaccine.

Understanding the Biology at Play
SARS-CoV-2 enters host cells via angiotensin-converting enzyme 2 (ACE2), which is expressed in thyroid tissue. Infection-related immune activation and potential disruption of the hypothalamic–pituitary–thyroid axis may transiently alter thyroid hormone regulation. Free triiodothyronine (FT3), the metabolically active thyroid hormone, is particularly sensitive to systemic stress and recovery states, making it a useful marker in this context.

The most notable changes occurred in patients with HT. By 90 days post-infection, HT patients demonstrated a statistically significant decrease in thyroid-stimulating hormone (TSH) compared with pre-infection levels. At the same time, FT3 levels significantly rose, with increases observed at both D30 and D90. These shifts suggest a delayed but measurable alteration in thyroid hormone dynamics following infection.

In contrast, patients with non-HT hypothyroidism showed fewer changes. The only significant finding in this group was a modest increase in FT3 at D30 compared with baseline (p = 0.017), with no significant changes in TSH or other thyroid parameters over time.

Importantly, free thyroxine (FT4), total T3, total T4, and thyroid antibody levels remained largely stable in both groups, indicating that COVID-19 did not trigger broad biochemical thyroid destabilization in most patients.

Vaccination and Thyroid Stability
Vaccination status emerged as a relevant modifier. In both the HT and non-HT groups, vaccinated patients exhibited relatively stable thyroid hormone levels before and after infection. Unvaccinated HT patients, however, showed greater variability, including a significant rise in TSH at D90.

While infection rates were numerically lower among vaccinated patients, this difference did not reach statistical significance.

Clinical Laboratory Changes Beyond Thyroid Hormones
Among non-thyroid markers, hemoglobin levels increased modestly after infection in HT patients.

Other indices, including lactate dehydrogenase, lipid parameters, lipoprotein(a), and vitamin D, did not show significant post-infection changes in either group.

Clinical Perspective
Taken together, these findings suggest that COVID-19 can produce mild, time-limited thyroid hormone fluctuations in patients with hypothyroidism—particularly those with autoimmune disease. From a clinical standpoint, the data support continued routine monitoring rather than preemptive treatment changes, while reinforcing the stabilizing role of vaccination in this population.

Reference
Li B, Feng X, Zhang Y, et al. Effect of COVID-19 infection on thyroid function status and clinical indexes among hypothyroid outpatients. Virulence. 2025;16(1):2441397. doi:10.1080/21505594.2024.2441397