Free T3 'optimal ranges': what the evidence actually supports

What did @mccallmcpherson actually say?

McCall McPherson told her 31.8K viewers to check their free T3, dismissed total T3 as "completely useless," and set a specific target range of 3.6 to 4.2 pg/mL for free T3. She added that people on T3 medication can "peak above 4.2 reasonably with little to no negative impact."

This is a confident, specific prescription delivered to a mass audience. The range she gives is narrower and higher than standard lab reference ranges, which typically run from about 2.3 to 4.2 pg/mL depending on the assay and lab. She's essentially telling viewers that the bottom half of normal is unacceptable. That's a clinical opinion, not a consensus fact, and it deserves scrutiny.

She's also speaking to people with Hashimoto's and hypothyroidism, populations that may already be anxious about labs and prone to self-adjusting medication based on social media guidance. That context matters.

Does the science back this up?

Partly, but not cleanly. The preference for free T3 over total T3 has reasonable support. The specific numerical target range she gives does not have strong published evidence behind it.

Free T3 is the biologically active form of thyroid hormone, and measuring it makes more physiological sense than total T3, which includes protein-bound hormone that cells cannot use. That part is defensible. Research by Hoermann et al. (2019, Frontiers in Endocrinology) found that free T3 is a stronger predictor of symptoms and metabolic outcomes than TSH or free T4 in some treated hypothyroid patients, lending some credence to focusing on free T3.

However, the claim that total T3 is "completely useless" is an overstatement. Total T3 remains clinically relevant in diagnosing hyperthyroidism, T3 toxicosis, and monitoring certain conditions. The American Thyroid Association has not abandoned it.

The 3.6 to 4.2 range she cites is not drawn from any published guideline. Standard reference ranges vary significantly across assays, and no major endocrinology organization has endorsed this specific window as a universal target.

What did they get wrong (or right)?

Right: Free T3 is clinically meaningful and often underevaluated. Wrong: Several things.

Dismissing total T3 as "completely useless" is simply inaccurate. It has specific clinical applications, particularly in hyperthyroidism evaluation. The ATA and European Thyroid Association still reference it in their diagnostic frameworks.

The 3.6 to 4.2 pg/mL target range appears to be a practitioner preference, not a guideline-backed standard. The lower boundary of 3.6 would classify a large portion of clinically healthy, symptom-free people as undertreated. Jonklaas et al. (2014, Thyroid) noted that optimal free T3 targets in treated hypothyroidism remain an area of active debate with no consensus threshold.

The statement that peaking above 4.2 on T3 medication causes "little to no negative impact" is also concerning. Supraphysiologic free T3 levels are associated with cardiac risks, including atrial fibrillation and bone density loss, as documented by Klein and Danzi (2016, New England Journal of Medicine). Saying this casually to a lay audience with no caveats is irresponsible.

What should you actually know?

Free T3 testing is underused in standard hypothyroid care, and that is a legitimate criticism of conventional practice. But the specific numbers McPherson gives are not sourced from clinical trials or guidelines. They reflect a functional medicine perspective that is popular but not validated at scale.

If your free T3 sits at 3.2 and you feel fine, you do not necessarily need intervention based on this video. If you feel symptomatic with a "normal" TSH and low-normal free T3, that is worth a real conversation with a clinician who can review your full panel, symptoms, and history, not a TikTok range.

People on T3-containing medications like liothyronine or desiccated thyroid do experience peak-and-trough fluctuations, which is a real pharmacokinetic issue. But the claim that exceeding 4.2 at peak is broadly safe does not account for individual cardiac risk, age, or comorbidities. This is not a one-size number.

The takeaway is not that McPherson is entirely wrong about thyroid labs being underinterpreted. It is that she is presenting a specific, unvalidated numerical target as settled fact to people who may act on it without clinical supervision.