Supplementary Material for: Investigation of the Relationship between MR-Based Supraclavicular Fat Fraction and Thyroid Hormones

<b><i>Purpose:</i></b> Brown adipose tissue (BAT) plays a potential role in energy and glucose metabolism in humans. Thyroid hormones (TH) are main regulators of BAT development and function. However, it remains unknown how the magnetic resonance (MR)-based proton density fat fraction (PDFF) of supraclavicular adipose tissue used as a surrogate marker for BAT presence relates to TH. Therefore, the purpose of this analysis was to investigate the relationship between supraclavicular PDFF and serum levels of TH. <b><i>Methods:</i></b> In total, 96 adult volunteers from a large cross-sectional study who underwent additional MR examination of the neck and pelvis were included in this analysis. Segmented PDFF maps of the supraclavicular and gluteal subcutaneous adipose tissue were generated. Delta PDFF was calculated as the difference between gluteal and supraclavicular PDFF and grouped as high (≥12%) or low (&lt;12%) based on the median and the clinical rationale of a high versus low probability of BAT being present. Thyroid-stimulating hormone (mIU/L), free triiodothyronine (FT3, pg/mL) and free thyroxine (FT4, ng/dL) levels were determined in blood samples. Body mass index (BMI) was calculated as weight (kg)/height (m)². Statistical analyses included the use of paired samples <i>t</i>test, simple linear regression analysis and a multivariable linear regression analysis. <b><i>Results:</i></b> The median age of the subjects (77% female) was 33 years, BMI ranged from 17.2 to 43.1 kg/m². Supraclavicular and gluteal PDFF differed significantly (76.5 ± 4.8 vs. 89.4 ± 3.5 %, <i>p</i> &lt; 0.01). Supraclavicular PDFF was associated with FT3 in subjects with high delta PDFF (<i>R</i>² = 0.17, <i>p</i> &lt; 0.01), with higher FT3 being associated with lower supraclavicular PDFF (<i>y</i> = 85.2 + –3.6 <i>x</i>). In a multivariable linear regression analysis considering further potential prognostic factors, the interaction between the delta PDFF group and FT3 remained a predictor for supraclavicular PDFF (B = –4.65, <i>p</i> &lt; 0.01). <b><i>Discussion/Conclusions:</i></b> Supraclavicular PDFF corresponds to the presence of BAT. In the present analysis, supraclavicular PDFF is correlated with FT3 in subjects with high delta PDFF. Therefore, the present findings suggest that biologically active T3 may be involved in the development of supraclavicular BAT.