Does zinc deficiency impair thyroid hormone signaling?

Based on the scientific evidence, zinc is structurally and functionally essential for thyroid hormone signaling, though the pathway through which nutritional deficiency impairs this system is primarily indirect.

Clinical and physiological evidence

• Zinc-finger structure of thyroid receptors: Thyroid hormone receptors ($\text{TR}\alpha$ and $\text{TR}\beta$) belong to the nuclear receptor superfamily and structurally contain two C4 zinc-finger motifs within their DNA-binding domains. Each receptor monomer coordinates two zinc ions ($Zn^{2+}$) to stabilize the helical structure required for binding to thyroid hormone response elements (TREs) on target DNA.
• Impact of zinc deficiency: While the physical dissociation of structurally bound zinc from thyroid receptors under dietary deficiency is difficult to demonstrate directly in vivo, nutritional zinc deficiency is strongly linked to clinical thyroid dysfunction. Experimental and clinical studies show that zinc-depleted states reduce circulating thyroid hormone levels (total $T_3$ and $T_4$) and can impair the peripheral conversion of $T_4$ to active $T_3$ by limiting deiodinase enzyme activity.

Downstream signaling and transcriptional impacts

• Transcriptional impairment: Canonical thyroid hormone signaling is fundamentally a transcriptional process. In the absence of sufficient $T_3$ or if the receptor's DNA-binding affinity is compromised, the receptor fails to transition from a repressor state to an activator state. This impairs the recruitment of critical co-activators and blocks $T_3$-dependent gene transcription.
• Indirect pathway disruption: In addition to potential structural vulnerabilities of the zinc-finger domain, zinc deficiency disrupts hypothalamic-pituitary-thyroid (HPT) axis regulation, lowering thyrotropin-releasing hormone (TRH) and thyroid-stimulating hormone (TSH) levels, which indirectly diminishes receptor activation due to inadequate ligand ($T_3$) availability.

Bottom line

Zinc is structurally required for thyroid hormone receptor DNA binding and downstream transcription. While a direct structural failure of the receptor during nutritional zinc deficiency remains a mechanistic hypothesis, clinical and animal data confirm that zinc deficiency impairs thyroid hormone signaling primarily by reducing thyroid hormone levels, disrupting the HPT axis, and suppressing $T_3$-dependent gene expression.