I've clarified this with Regenerus after they contacted the lab in Switzerland that carries out the test. Here is a summary of the DIO2 enzyme, the genetic test and how the results are reported by Regenerus:
There are three deiodinase enzymes D1, D2 and D3 controlled by genes DIO1, DIO2 and DIO3. D2 converts T4 to T3 and is responsible for local T3 generation in organs such as the pituitary and brain.
A DIO2 single nucleotide polymorphism (SNP) rs225014 is associated with impaired baseline psychological wellbeing and enhanced response to combined T4 / T3 therapy academic.oup.com/jcem/artic... . In this SNP the possible base combinations of thymine (T) and cytosine (C) are TT, TC and CC. Patients with the CC base pair have impaired psychological wellbeing and respond to combined T4 / T3 therapy. i.e. patients who have inherited the C allele from BOTH parents.
Note that there is no difference in T3 levels between subjects with or without the polymorphism, probably because D2 activity in the brain controls local T3 levels not circulating T3. Patients who think this polymorphism is causing low serum fT3 levels are mistaken. Blood T3 levels are not changed by this polymorphism.
Now to explain the confusion!
Rs225014 is also known as Thr92Ala. The T allele encodes the Thr (threonine) and the less common C allele encodes the Ala (alanine). You can report TT, TC and CC or you can report ThrThr, ThrAla and AlaAla, depending on whether you report the base pair or the resultant amino acids.
Regenerus report the amino acids. Unfortunately, they incorrectly describe them as alleles and use the abbreviations T and A instead of Thr and Ala. This becomes even more confusing as there are four types of bases found in a DNA molecule: adenine (A), cytosine (C), guanine (G), and thymine (T).
So, where Regenerus report a T it corresponds to Thr which comes from a T allele and where Regenerus report an A it corresponds to Ala which comes from a C allele.
Patients with the CC alleles have impaired baseline psychological wellbeing and respond to combined T4 / T3 therapy. This corresponds to an AA result on the Regenerus report.