The basic ideas are:

1. TSH is a measure of pituitary function, NOT thyroid function. Therefore TSH is not utilized to assess the body’s (“peripheral”) need for thyroid hormone.
2. If only T4 is adjusted, there are only three possible changes, up, or none. If T3 and T4 can be adjusted, there are nine possible changes.
3. By measuring just three parameters, Free T4, Free T3 and Reverse T3, each with high, low, or “optimal”, there are 27 possible scenarios.

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Proof of Components

rT3 blocks production of T3

!A Study of Extrathyroidal Conversion of T4 to T3.1976.r

Cortisol diverts T4 to rT3 rather than T3

Neuroendocrine System Regulatory Mechanisms–Acute-Coronary-Syndrome.2018

Nonthyroidal Illness Syndrome Across the Ages.2019

TSH measures pituitary, not thyroid function

Coordination-of-hypothalamic-and-pituitary-T3-production-regulates-TSH-expression.2013.r.pdf

Thyroid Hormone is Consumed by Immune Cells During Infection

Which means that the hypothalamus and pituitary (which determine TSH level) may not respond properly during certain types of infections and deficiencies (for more click here).

Articles Supporting the Need/Benefit of T3 

Low-T3 Syndrome- A Strong Prognostic Predictor of Death in Patients With Heart Disease.2003

Interactive Effects of Apolipoprotein E e4 and Triiodothyronine on Memory Performance in Patients With Subjective Cognitive Decline.2020

A Systematic Review and Meta-Analysis of Patient Preferences for Combination Thyroid Hormone Treatment for Hypothyroidism.2019.r

The Swinging Pendulum in Treatment for Hypothyroidism–From (and Toward) Combination Therapy.2019.r

Thyroid Hormone Therapy for Older Adults.2017.nejm

For more information about the benefits and indications for T3, see Combined Thyroid T4-T3 Therapy