Effects of Hyperthyroidism on Adipose Tissue Activity and Distribution in Adults

Area of Science:

• Endocrinology
• Metabolic Research
• Nuclear Medicine

Background:

• Brown adipose tissue (BAT) is metabolically active in adult humans.
• Thyroid hormones (TH) influence BAT thermogenesis through central and peripheral pathways.
• The precise impact of hyperthyroidism on human BAT activity and volume remains unclear.

Purpose of the Study:

• To investigate the effect of TH on the metabolic activity of brown and white adipose tissue (WAT).
• To assess the impact of TH on abdominal visceral and subcutaneous adipose tissue areas.
• To examine changes in serum levels of metabolically active cytokines in hyperthyroidism.

Main Methods:

• Nineteen patients with overt hyperthyroidism underwent repeated 2-[18F]fluoro-2-deoxy-d-glucose positron emission tomography/computed tomography (2-[18F]FDG PET/CT) scans.
• Scans were performed in both hyperthyroid and euthyroid states to measure 2-[18F]FDG uptake (metabolic activity).
• Abdominal fat areas were quantified via CT segmentation, and serum cytokine levels were analyzed.

Main Results:

• 2-[18F]FDG uptake was significantly increased in BAT during the hyperthyroid state compared to the euthyroid state.
• No correlation was found between serum free triiodothyronine (fT3) or free thyroxine (fT4) levels and 2-[18F]FDG uptake in BAT or WAT.
• Hyperthyroidism was associated with decreased visceral adipose tissue and skeletal muscle areas, and increased serum adipocyte fatty acid-binding protein (AFABP) levels.

Conclusions:

• Increased TH levels have a limited direct effect on BAT and WAT glucose uptake compared to skeletal muscle.
• Hyperthyroidism leads to a reduction in visceral adipose tissue and an increase in AFABP levels.
• Further research is needed to fully elucidate the complex interplay between TH, adipose tissue, and metabolic regulation.