Introduction

Thyroid swellings range from benign colloid goiter and inflammatory thyroiditis to malignant neoplasms and are among the most common endocrine disorders encountered clinically. Palpable nodules are identified in only a modest proportion of adults, but high-resolution ultrasonography (USG) detects subclinical nodules far more often, particularly in women and older individuals. Thyroid carcinoma is the leading endocrine malignancy worldwide, and its recorded incidence has risen substantially in recent decades, a trend driven in part by wider use of high-resolution imaging rather than a true increase in disease burden alone [1].

The core diagnostic problem in nodular thyroid disease is separating the small fraction of clinically important malignant nodules from a much larger benign population so that unnecessary thyroidectomy is avoided while malignant disease is still caught and treated promptly. Preoperative evaluation has accordingly become a multimodal pathway combining clinical assessment, thyroid function testing, USG with standardized risk scoring, and cytopathological sampling by fine-needle aspiration [2].

Among available preoperative tests, fine-needle aspiration cytology (FNAC) is favored for its low cost, minimal invasiveness, and extensive prior validation. Cytological findings are reported using the Bethesda System for Reporting Thyroid Cytopathology, which assigns each specimen to one of the six categories, each linked to an estimated malignancy risk that guides subsequent management [3]. Reported sensitivity and specificity of FNAC vary across published series, reflecting differences in operator experience, sampling technique, and case mix, particularly the proportion of follicular-patterned lesions [4,5].

USG provides a complementary structural assessment, most often standardized through the American College of Radiology (ACR) Thyroid Imaging Reporting and Data System (TI-RADS), in which nodules are scored based on five sonographic features - composition, echogenicity, shape, margin, and echogenic foci - to yield a final category ranging from TR1 to TR5 [6]. Serum thyroid-stimulating hormone (TSH) is routinely measured as part of initial evaluation; a lower serum TSH has been loosely associated with higher malignancy risk in some epidemiological data, though this relationship is inconsistent enough that TSH alone cannot reliably discriminate malignancy in an individual patient [7]. Definitive diagnosis still rests on the histopathological assessment of resected tissue, now classified according to the 2022 World Health Organization (WHO) Classification of Thyroid Neoplasms, which reorganized follicular-patterned lesions and recognized non-invasive follicular thyroid neoplasm with papillary-like nuclear features (NIFTP) as a distinct borderline entity [8].

FNAC, USG TI-RADS, and thyroid function testing have each been extensively studied in isolation, but correlative studies evaluating all three together against histopathology in one cohort remain comparatively scarce, particularly in the Indian subcontinent. This study addresses that gap. Its primary aim was to examine the multimodal diagnostic correlation among FNAC using the Bethesda System, USG using ACR TI-RADS, serum TSH status, and histopathology. Its secondary aims were to quantify the individual diagnostic performance of FNAC and USG and their inter-modality agreement with HPE.

The study objectives are as follows:

Primary objective: To evaluate the diagnostic performance and correlation of FNAC using the Bethesda System, USG using ACR TI-RADS, and serum TSH status against histopathological examination as the reference standard.

Secondary objectives: To quantify the individual diagnostic performance of FNAC and USG (ACR TI-RADS) and assess their agreement with histopathological findings.