Research Blog

November 12, 2022

Thyroid Biomarkers: Free T3

Optimal Takeaways

Free T3 (FT3) is the most biologically active form of thyroid hormone as it is unbound and readily available. Initial measurement of FT3 may not reveal early hypothyroidism but it is clinically useful for monitoring progress and assessing symptomatology. Low free T3 can also be seen with euthyroid sick syndrome, decreased calorie intake, diabetes, heart failure, and pulmonary and liver disease. Elevated FT3 is associated with hyperthyroidism, decreased lean body mass, and the use of certain medications.

Conventional Lab Range: 2.30 – 4.20 pg/mL (3.53 – 6.45 pmol/L)

Optimal Dx’s Optimal Range: 3.00 – 3.50 pg/mL (4.61 – 5.38 pmol/L)

Low free T3 may be seen with hypothyroidism, euthyroid sick syndrome (non-thyroidal illness syndrome), calorie deprivation, heart failure, liver disease, diabetes, pulmonary disease (Moura 2016), elevated inflammatory cytokines, hypoxia (Ataoglu 2018), and phthalate exposure (Choi 2020).

High free T3 may be seen with hyperthyroidism as well as psychiatric illness (Moura 2016), insulin resistance, and decreased lean body mass (Roef 2012). Heparin can increase serum free fatty acids which can dislodge thyroid hormones from their binding proteins and increase FT3 (Koulouri 2013).


Serum free triiodothyronine (FT3) reflects the amount of biologically available T3 in the blood and may reflect intracellular concentrations to some degree as well (Abdalla 2014). Evaluation of FT3 may be most valuable for assessing clinical progress and response to therapy versus an initial diagnosis of hypothyroidism as T3 and FT3 levels may be maintained within normal limits initially due elevated TSH and upregulation of the conversion of T4 to T3 (Garber 2012). Along with iodine and selenium, zinc also plays an integral role in thyroid hormone metabolism and is required for the conversion of T4 to T3. One 12-week study demonstrated that zinc supplementation significantly increased FT3 levels as well as the FT3 to FT4 ratio in hypothyroid patients (McGregor 2015).

A decrease in FT3 is considered a sensitive indicator of both acute and chronic disease, especially in the elderly, and can be associated with mortality (Ataoglu 2018). A decreasing FT3 in the absence of known thyroid disease may be caused by euthyroid sick syndrome, also known as low T3 syndrome or non-thyroidal illness syndrome (NTIS). The phenomenon may be associated with transient changes in deiodinase enzymes or abnormalities in thyroid binding globulin. The syndrome may be accompanied by elevated proinflammatory cytokines and a low or normal TSH though TSH can also be slightly increased as well (Moura 2016). In addition, euthyroid sick syndrome is characterized by an increased reverse T3, normal/high free T4, and normal total T4 (Malik 2002).

Lower levels of free T3 within the conventional range may be associated with symptomology in those on T4 monotherapy despite a low TSH. A retrospective analysis of 319 thyroid cancer patients observed that those on monotherapy whose FT3 remained in the lower half of the reference range continued to complain of common hypothyroid symptoms including fatigue, intolerance to cold, and weight gain. Symptoms were not resolved until FT3 increased to at least the upper half of the conventional reference range and TSH was below the conventional reference range (Larisch 2018). Researchers note that monotherapy often results in FT3 levels below the median, even below the reference range, and that the clinical consequences should be assessed for and addressed. A trial and monitoring of combination therapy with T4 and T3 may be effective unless contraindicated (Ettleson 2020).

Lower free T3 was observed in patients with severe COVID-19 and coincided with increased all-cause mortality. Meta-analysis of seven retrospective studies comprising 1,183 hospitalized COVID patients found that lower FT3 was associated with ICU admission, severity of disease, and mortality. Researchers note that the cytokine storm observed in severe COVID may be the cause of decreased FT3 and subsequent clinical deterioration (Llamas 2021).

Elevations in free T3 must be investigated as well. Some research reports an association of higher FT3 with higher BMI, increased insulin resistance, and lower lean body mass, though researchers recommend further evaluation of this clinical presentation (Roef 2012).

One study investigating the incidence of thyroid disease in breast cancer observed higher levels of FT3, FT4, TSH, and TPO antibodies in those with cancer versus controls. Mean serum FT3 in patients was twice that of controls, i.e., 4.71 pg/mL (7.25 pmol/L) versus 2.22 pg/mL (3.42 pmol/L) respectively. Mean TSH in patients was 4.12 versus 1.39 uU/mL in controls. Some research suggests that altered thyroid function may influence the progression of breast cancer (Ali 2011).

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Tag(s): Biomarkers

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