Context: Brown adipose tissue (BAT) generates heat during adaptive thermogenesis in response to cold temperature. Thyroid hormone (TH) receptors, type 2 deiodinase, and TSH receptors are present on brown adipocytes, indicating that the thyroid axis regulates BAT. It is unknown whether absent TH in humans would down-regulate development of BAT and its thermogenic function.
Objective: The objective of the study was to examine BAT by magnetic resonance imaging (MRI) and infrared thermal imaging (IRT) in a pediatric patient with severe primary hypothyroidism before and after TH treatment.
Design/setting: This study was a case report with longitudinal follow-up in a tertiary center.
Main outcome measures: BAT fat fraction (FF) by MRI and skin temperature by IRT were measured.
Results: An 11.5-year-old female was severely hypothyroid (TSH, 989 μIU/mL; free T4, 0.10 ng/dL; low thyroglobulin, 3.0 ng/mL). Low MRI measures of FF (56.1% ± 3.7%) indicated that BAT was abundantly present in the supraclavicular fossa. IRT showed higher supraclavicular temperature (36.0°C ±0.16°C) than the suprasternal area (34.3°C ± 0.19°C). After 2 months of TH replacement, she was euthyroid (TSH, 4.3 μIU/mL; free T4, 1.49 ng/dL; T3, 102 ng/dL) at which time supraclavicular BAT decreased (increased FF 60.7% ± 3.8%). IRT showed a higher, more homogeneous skin temperature throughout the upper thorax (supraclavicular, 37.1°C ± 0.23°C; suprasternal, 36.4°C ± 0.13°C). The overall size of the supraclavicular fat depot decreased from 84.79 cm(3) to 41.21 cm(3).
Conclusions: These findings document the presence of BAT and thermogenesis in profound hypothyroidism and suggest a role for TSH and/or TRH as a potential regulator of BAT.