Are selenium and zinc essential for T4→T3 conversion and thyroid receptor function?

Clinical and metabolic evidence

• Selenium-dependent conversion: The conversion of thyroxine ($T_4$) to the metabolically active triiodothyronine ($T_3$) is catalyzed by iodothyronine deiodinase enzymes (specifically types 1 and 2, or DIO1 and DIO2). These enzymes are selenoproteins, meaning selenium is an obligate structural component of their active sites in the form of the amino acid selenocysteine.
• Clinical deficiency markers: In states of severe selenium deficiency, deiodinase activity is significantly compromised, leading to an impaired conversion pathway. This is clinically characterized by a decreased $FT_3$ to $FT_4$ ratio, marked by elevated free $T_4$ and reduced free $T_3$.
• Zinc-dependent thyroid regulation: Zinc status directly influences systemic thyroid hormone metabolism. Clinical and observational studies demonstrate that zinc deficiency is strongly associated with a hypothyroid-like shift, presenting with decreased active $T_3$, reduced free $T_4$, and elevated thyroid-stimulating hormone (TSH). Supplementing zinc in deficient individuals has been shown to successfully restore and normalize these peripheral thyroid hormone levels.

Mechanistic explanations

• Selenocysteine incorporation: The synthesis of DIO1 and DIO2 requires a highly specialized translational process. A selenocysteine insertion sequence (SECIS) element in the $3'$ untranslated region of the deiodinase mRNA recodes an in-frame UGA stop codon, allowing the direct incorporation of selenocysteine. This process relies entirely on sufficient intracellular bioavailable selenium.
• Receptor-level zinc fingers: At the nuclear level, thyroid hormone receptors (TRs) belong to the nuclear hormone receptor superfamily and require zinc to function. The DNA-binding domain of the TR utilizes a highly conserved $Cys_2\text{-}Cys_2$ zinc-finger motif. Zinc ions physically stabilize this architecture, which is a structural prerequisite for the receptor to bind to thyroid hormone response elements (TREs) on DNA and initiate gene transcription.
• Transcription factor interaction: Beyond the receptor itself, several zinc-finger-containing transcription factors and coregulators (including GATA2, GLIS3, and CTCF) cooperate with thyroid hormone receptors to modulate thyroid hormone synthesis and downstream metabolic transcription.

Bottom line

Both selenium and zinc are physiologically essential for normal thyroid function. Selenium is a mandatory structural component of the deiodinase enzymes that convert $T_4$ to active $T_3$, while zinc acts both as a metabolic regulator of this conversion and as a critical structural partner stabilizing the zinc-finger domains of thyroid hormone receptors to allow proper gene expression.