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Full Thyroid Panel: TSH, Free T4, Free T3, Reverse T3

Published: 7/3/2025

Full Thyroid Panel: TSH, Free T4, Free T3, Reverse T3

There is no more critical blood panel for assessing your core metabolic rate than a full thyroid panel. A standard screening that only looks at Thyroid-Stimulating Hormone (TSH) is practically useless and is the primary reason so many cases of functional hypothyroidism go undiagnosed for years. TSH only tells you what the pituitary gland is asking the thyroid to do; it tells you nothing about the thyroid's response or what is happening to the hormones in the rest of the body.

To get a complete picture, you must insist on measuring all four key markers. The goal is not just to be "within the normal range," but to be in the optimal part of that range, which signifies a high-energy, pro-metabolic state.

1. TSH (Thyroid-Stimulating Hormone)

  • What it is: The signal from the pituitary gland telling the thyroid to produce more hormone.

  • Optimal Range: While conventional labs often accept a TSH up to 4.5 or 5.0, this is far too high. A healthy, pro-metabolic TSH should be low. As Dr. Ray Peat advocated, it is best if your TSH is below 1.0, and ideally as low as 0.4. A high TSH is a clear sign that the brain is screaming for more thyroid hormone because the body is not getting enough.

2. Free T4 (Thyroxine)

  • What it is: The primary, but largely inactive, storage hormone produced by the thyroid gland.

  • Optimal Range: You want your Free T4 to be in the upper quartile (top 25%) of the lab's reference range. This indicates your thyroid gland is producing a robust amount of raw material.

3. Free T3 (Triiodothyronine)

  • What it is: The active, powerhouse hormone that drives your metabolic rate. This is the most important marker of all.

  • Optimal Range: Like Free T4, you want your Free T3 to be high in the upper quartile of the reference range. A low Free T3, even with a normal T4, is a definitive sign of poor T4-to-T3 conversion and functional hypothyroidism.

4. Reverse T3 (rT3)

  • What it is: The anti-thyroid, "emergency brake" hormone produced under conditions of stress, illness, or starvation.

  • Optimal Range: You want your rT3 to be as low as possible. A high rT3 level is a major red flag, indicating that your body is actively shutting down your metabolism by shunting T4 away from its active T3 pathway.

Interpreting the Panel as a Whole

A single one of these markers in isolation is not enough. The complete picture tells a story:

  • The Goal State (High Metabolism): Low TSH, High Free T4, High Free T3, Low rT3.

  • Classic Hypothyroidism: High TSH, Low Free T4.

  • Poor Conversion / Euthyroid Sick Syndrome: "Normal" TSH and T4, but Low Free T3 and/or High rT3. This is the pattern most often missed by standard testing.

Insisting on this full panel is a non-negotiable step in taking control of your health. It is the only way to truly know if your metabolic engine is receiving the "go" signal it needs to thrive.

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