Does low hepatic T3 signaling raise LDL cholesterol by reducing LDL receptor expression?

Thyroid hormones play an essential role in cardiovascular and metabolic homeostasis, and clinical observations have long linked hypothyroid states to dyslipidemia. This relationship is primarily driven by active thyroid hormone signaling in the liver.

Mechanistic explanations

• Transcriptional regulation: Triiodothyronine ($T_3$) directly controls the transcription of the low-density lipoprotein receptor (LDLR) gene in hepatocytes. In healthy states, $T_3$ binds to the thyroid hormone receptor beta-1 ($TR\beta1$), which heterodimerizes with the retinoid X receptor alpha ($RXR\alpha$). This complex binds to specific thyroid hormone response elements (TREs) on the LDLR promoter, stimulating gene expression.
• Receptor expression and clearance: When hepatic $T_3$ signaling is diminished, this direct transcriptional activation is lost. The resulting reduction in hepatic LDLR mRNA and protein expression limits the number of functional receptors on the hepatocyte surface. Because receptor-mediated endocytosis is the primary pathway for clearing low-density lipoprotein (LDL) from the blood, a shortage of these receptors slows the fractional catabolic rate of LDL.

Clinical evidence and implications

• Circulating cholesterol accumulation: With slower LDL clearance kinetics and relatively constant hepatic lipoprotein production, LDL particles accumulate in the systemic circulation. This directly translates to elevated serum LDL cholesterol levels.
• Therapeutic relevance: In clinical models, normalizing $T_3$ levels or selectively stimulating hepatic $TR\beta$ restores the binding of transcription factors to the LDLR promoter, rescuing receptor expression and accelerating the clearance of circulating LDL to normal levels.

Bottom line

• Low hepatic $T_3$ signaling downregulates hepatic LDL receptor transcription via impaired $TR\beta1$/$RXR\alpha$ activation, reducing receptor density on hepatocytes. This directly slows the clearance of LDL particles from the bloodstream, leading to an increase in circulating LDL cholesterol.