Ruiz-Núñez et al’s 2018 article, “Higher Prevalence of “Low T3 Syndrome in Patients With Chronic Fatigue Syndrome,” breaks down the common misconception that Low T3 syndrome only occurs in critical illness.
Low T3 can also occur in a chronic state during chronic illness.
A more extreme state of free T3 below reference range occurred in 16% (16 out of 98) Chronic Fatigue Syndrome (CFS) patients versus 7% (7 out of 99) controls, yielding an odds ratio of 2.56 for having low Free T3 in CFS.
Many of the CFS patients’ thyroid hormone levels and TSH were similar to those seen in Low T3 syndrome: The TSH was not abnormal, but Free T3 and T4 were both lower and Reverse T3 was higher.
However, if their methods had not compared the incidence of Low T3, they would not likely have found it, because the average Free T3 level across both cohorts (CFS vs healthy) was 5.2 pmol/L.
It just goes to show that statistical averages can be deceptive in thyroid research. You have to look for diversity within the cohort to see meaningful patterns, as these researchers did.
Even though their study excluded anyone on thyroid medication, the researchers did not overlook the similar pattern of low Free T3 levels in thyroid patients.
Later in the article is a discussion section with this heading:
“Comparison With Hypothyroid Patients Treated With T4 Monotherapy”
This is an important section of the article because it dispels a similar common medical misconception — the false idea that chronic Low T3 Syndrome is not a cause for concern in T4 therapy.
In this section, they state
“The encountered “low T3 syndrome” in our study resembles the thyroid hormone profile of a subgroup of hypothyroid patients receiving T4 monotherapy.
Substitution with T4 is the currently recommended treatment of hypothyroid patients, like those with Hashimoto thyroiditis.
Nevertheless, it is becoming increasingly clear that a subgroup of these patients experiences residual hypothyroid symptoms, including psychological and metabolic traces.
These symptoms occur despite reaching a chemical euthyroid state, i.e., normal TSH. […]
In so-called “euthyroid, yet symptomatic” patients, the basal metabolic rate and serum cholesterol, among others, are not fully restored and they are also likely to have both low TT3 and FT3.”
Is a lower Free T3 significant? Yes.
Their review of research explains what lower Free T3 levels can mean for the body’s peripheral T3 levels:
“A recent study with chemically induced hypothyroidism in rats showed a more severely reduced tissue T3 than serum FT3, averaging 1–6% of the baseline versus 30%, respectively.
In addition, the extent of tissue T3 reduction, expressed as percentage of the baseline, was not homogeneous, with more serious reductions occurring in the order: liver = kidney > brain > heart > adipose tissue (90) [Liver, kidney, and brain can suffer larger T3 deficits compared to heart and fat tissue].
In other words, the finding of slightly decreased circulating FT3 and perhaps also FT3 levels in the lower reference range may reflect the tip of the iceberg of the genuine T3 deficits in target tissues.”