A Tissue the Body Keeps Especially Well Supplied

Why This Particular Tissue

No other organ in the body holds as much selenium per gram of tissue as the gland at the base of the neck. This observation comes from older analyses of organ tissue and has served for decades as the starting point for asking why this trace element is needed there in such concentration.

Animal studies in which selenium intake was artificially and severely restricted showed a striking pattern: while selenium content in the liver and muscle dropped off quickly, it stayed comparatively stable in this one tissue for much longer. Researchers describe this as the body serving certain tissues first when supply runs short — a distribution pattern that this organ apparently benefits from more than most.

Europe's food-safety authority, EFSA, reviewed the science behind this connection, and on that basis the European Commission authorized a health-related claim on selenium tied to this tissue. The sections below set out which enzyme systems sit behind that prioritization.

Two Enzyme Families, Two Sites of Action

Building the hormone precursors inside this tissue's storage vesicles continuously generates hydrogen peroxide as an unavoidable byproduct — a molecule that could damage the surrounding cell structures if it lingered too long. Two separate, selenium-dependent enzyme groups continuously take care of neutralizing it, though at different locations.

The first group works directly inside the cell, right where the peroxide forms. The second, a version of the same enzyme type that circulates in blood plasma, takes on a similar job outside the cell, in the immediate vicinity of the tissue. Both versions are selenoproteins — proteins that carry selenium built firmly into their active center — and both have been described independently in the scientific literature.

Without enough selenium, neither enzyme group can be built up to its full number, and the neutralizing of this byproduct slows down measurably. This is exactly the connection the EU-authorized sentence addresses:

“Selenium contributes to the normal thyroid function” — EU-authorized wording · Regulation (EU) No 432/2012

A Second Phase of Life Outside the Gland

The hormone molecules formed in this tissue haven't reached the end of their journey once they're released into the blood. Most of their actual activation happens later, in the liver, kidney, and other tissues, carried out by a group of enzymes that also depend on selenium as a cofactor: the deiodinases.

These enzymes remove a single iodine atom from the hormone molecule in a targeted way, converting it into the biologically more active form. A related enzyme from the same family can carry out that same step in the opposite direction instead, turning the molecule into a barely active variant. Which of the two forms dominates in a given tissue depends on the specific cell type — a fine-tuning system that sits outside this one organ but doesn't run at full capacity without selenium.

Iodine itself, as a building block of the hormone molecules, is also part of this chain; its role is the subject of a separate EU claim not covered here, and it's mentioned at this point purely for scientific completeness.