Thyroid echogenicity index in chronic autoimmune thyroiditis and its seronegative variant

Introduction: Seronegative chronic autoimmune thyroiditis (seronegative-CAT) remains a poorly defined clinical condition, and its relationship to classic chronic autoimmune thyroiditis (CAT) at the tissue level has yet to be fully characterized. The present study aimed to assess whether the quantitative thyroid echogenicity index (EI), computed by referencing the sternocleidomastoid muscle as an internal standard, can differentiate patients with classic CAT and seronegative-CAT from healthy individuals, and whether echoacoustic alterations differ between the two patient subgroups. Material and methods: This prospective study enrolled 28 patients diagnosed with CAT (group 1), 26 patients with seronegative-CAT (group 2), and 20 age- and sex-matched healthy volunteers (group 3). All sonographic evaluations were performed by a single experienced operator using a 9 MHz linear transducer. Echogenicity index was defined as the ratio of mean thyroid parenchymal echogenicity to mean sternocleidomastoid muscle echogenicity and was derived from archived transverse sonographic images using custom-developed measurement software. Between-group comparisons included EI, absolute thyroid echogenicity, gland volume, and thyroid function parameters. Results: Mean EI was significantly reduced in both the CAT group (1.41) and the seronegative-CAT group (1.57) relative to healthy controls (1.97) (overall p < 0.001; CAT vs. controls p < 0.001; seronegative-CAT vs. controls(overall p < 0.001; CAT vs. controls p < 0.001; seronegative-CAT vs. controls p = 0.005). No statistically significant difference in EI was identified between the two patient subgroups (p = 0.419). Absolute thyroid echogenicity was likewise significantly lower in the CAT group compared with controls (p < 0.05), with no significant difference observed between CAT and seronegative-CAT. Neck muscle echogenicity, gland volume, thyroid-stimulating hormone (TSH), and free thyroxine (fT4) values were comparable across all three groups. Conclusions: Quantitative EI reliably distinguishes both CAT and seronegative-CAT from healthy individuals; however, it does not reveal significant echoacoustic differences between the two patient subgroups. This observation supports the concept that autoimmune tissue-level alterations — manifested as reduced echogenicity — occur independently of serological status, reinforcing seronegative-CAT as a clinically distinct entity that converges with classic CAT at the ultrasonographic level.