Are T3 (triiodothyronine) and RT3 (reverse triiodothyronine) thyroid tests covered by the Medicare Benefits Schedule (MBS) in Australia for patients suspecting Small Intestine Bacterial Overgrowth (SIBO) as a root cause of their symptoms?

MBS Funding for T3 and RT3 Testing in Australia

T3 and RT3 thyroid tests are NOT routinely funded by Medicare Benefits Schedule (MBS) in Australia for the indication described (suspecting SIBO without thyroid symptoms), and there is no clinical evidence supporting their use in this context.

Clinical Context and Evidence Base

The scenario presents a patient requesting T3 and reverse T3 (RT3) testing based on suspected SIBO as a "root cause," despite having no recent thyroid symptoms or previous thyroid testing. This request lacks clinical justification on multiple levels:

Why These Tests Are Not Indicated

• TSH is the appropriate initial screening test for thyroid dysfunction, with high sensitivity (98%) and specificity (92%) when used to confirm suspected thyroid disease 1

• T3 testing has limited clinical utility even in confirmed thyroid disease - it is primarily used to distinguish subclinical hyperthyroidism from overt T3 toxicosis when TSH is suppressed (<0.1 mIU/L) and free T4 is normal 2

• RT3 testing has no established clinical utility in mainstream endocrinology practice, with systematic reviews finding little evidence to support its use 3

The RT3 Controversy

• RT3 is biologically inactive and was historically elevated in "euthyroid sick syndrome" (severe systemic illness), not in routine outpatient thyroid evaluation 4, 5

• Wide practice variation exists, with 95% of providers who order RT3 placing two orders or fewer, while a small subset (0.1% of providers, predominantly functional medicine practitioners) account for 29.5% of all RT3 orders 3

• No peer-reviewed evidence supports the functional medicine concept of "RT3 dominance" or using RT3 levels to guide T3 replacement therapy in symptomatic patients with normal TSH 5, 3

• Recent research even suggests RT3 may stimulate cancer cell proliferation, raising questions about its biological role 6

MBS Funding Reality in Australia

While I cannot access the current MBS schedule directly, the clinical evidence framework makes clear:

• MBS typically funds tests with established clinical utility based on evidence-based guidelines

• Thyroid testing is funded when clinically indicated - meaning symptoms or risk factors suggesting thyroid dysfunction must be present 1

• RT3 testing specifically lacks guideline support from major endocrine societies (American Thyroid Association, Endocrine Society, American College of Physicians) 1

• SIBO has no established connection to thyroid function that would justify thyroid testing in asymptomatic patients

Appropriate Clinical Approach

If thyroid dysfunction is genuinely suspected, the evidence-based approach is:

• Start with TSH measurement as the initial screening test 1

• Add free T4 if TSH is abnormal to distinguish primary from central thyroid disorders 7, 8

• Consider free T3 only in specific scenarios: when TSH is suppressed (<0.1 mIU/L) with normal free T4 to evaluate for T3 toxicosis 2

• Never order RT3 for routine thyroid evaluation - it has no role in clinical decision-making 3

Common Pitfalls to Avoid

• Do not order thyroid tests without clinical indication - symptoms of thyroid dysfunction or specific risk factors (elderly, postpartum, Down syndrome, high radiation exposure) should be present 1

• Do not rely on single TSH measurements - transient suppression occurs with various conditions including medications, non-thyroidal illness, and recovery from thyroiditis 7

• Do not use T3 or RT3 to diagnose hypothyroidism - TSH and free T4 are the essential tests 8

• Beware of overtreatment - there is good evidence that overtreatment with levothyroxine occurs in a substantial proportion of patients when testing is done without clear indication 1

Bottom Line for This Patient

Daniel's request for T3 and RT3 testing is not clinically justified and would not meet MBS funding criteria. The appropriate response is to:

• Explain that these tests are not indicated without thyroid symptoms or abnormal TSH
• If thyroid concerns exist, start with TSH testing only
• Address the SIBO concerns through appropriate gastrointestinal evaluation, which has no connection to thyroid hormone testing
• Avoid the functional medicine approach of using RT3 levels, which lacks evidence-based support 3