Did you know…
The enzymes that convert the thyroid prohormone T4 into active T3 are selenium-dependent. They cannot function without selenium. This means that a patient with adequate iodine, adequate T4 production, and a normal TSH can still be functionally hypothyroid at the tissue level if selenium is depleted. In ordinary thyroid workup, selenium status is almost never assessed.
The claim, in the reference of thyroid molecular biology
In 1991, Michael Berry, Zaman Banu, and P. Reed Larsen published in Nature the finding that the type I iodothyronine deiodinase, the enzyme responsible for most of the peripheral conversion of T4 to T3, is a selenoprotein. The paper opens the modern understanding of the selenium-thyroid axis:
“The mRNA for type I deiodinase contains a UGA codon for selenocysteine which is necessary for maximal enzyme activity.”
— Berry, Banu & Larsen, Nature, 1991
The biochemistry that follows from this finding is quantitatively striking. The higher nucleophilicity of selenium compared to the sulfur of cysteine makes the selenocysteine version of the enzyme roughly 100-fold more efficient at catalyzing the deiodination of iodothyronines than a cysteine-substituted version would be. Selenium is not a minor cofactor. It is the catalytic centre.
The clinical signature that ordinary workup misses
In selenium deficiency, the signature is: elevated T4, low T3, normal or normal-high TSH. A patient with this profile has a thyroid gland producing enough hormone, a pituitary satisfied by the amount, and a periphery that cannot activate what circulates. The T3 the tissues need does not arrive.
What routine care measures
The routine thyroid workup measures TSH, sometimes free T4, occasionally free T3. It does not measure selenium status. It does not measure the ratio of T3 to reverse T3, which would flag preferential shunting toward the inactive metabolite. A patient can spend years on levothyroxine replacement without the selenium question ever being asked.
This entry is a companion to Right to Optimal Hormonal Levels.