Check a TSH result against your lab’s reference range or a typical adult 0.4–4.0 mIU/L range, compare two TSH tests, and read µIU/mL as mIU/L.

Use the interval on the laboratory report. A range comparison is not a diagnosis or treatment target.

Use the range printed on your lab report when you have it; labs differ.

Measured Free T4 and research index

Related Calculators

TSH Formula

The following formulas are illustrative base-10 equations. Their coefficient pairs appear in Benhadi et al. (2010), but logarithm base and concentration units were not independently confirmed from the accessible abstract; base 10 and the displayed units are retained assumptions for this mathematical demonstration. The pilot study used 21 healthy volunteers aged 51–74, including single-dose T4 and T3 experiments, and could not accurately assess individual set points. These equations do not replace measured TSH, Free T4 or clinical review.

Equation 1 – FT4 illustration (coefficient pair reported by Benhadi et al., 2010)
log10(TSH) = 1.50 − 0.059 × FT4(pmol L−1)
⇒ TSH (mIU L−1) = 10[1.50 − 0.059 × (FT4(ng/dL) × 12.87)]

Equation 2 – Total T3 illustration (coefficient pair reported by Benhadi et al., 2010)
log10(TSH) = 0.790 − 0.245 × T3(nmol L−1)
⇒ TSH (mIU L−1) = 10[0.790 − 0.245 × (T3(ng/dL) × 0.01536)]

*These are cohort equations with limited individual applicability. Neither output receives a clinical range grade. Inverting the FT4 equation is an algebraic exercise, not a validated individual prediction. Use measured tests and discuss interpretation with a clinician.*

Variables:

  • TSH is the thyroid-stimulating hormone level (mIU/L).
  • FT4 is the free thyroxine level (ng/dL; sometimes reported as pmol/L).
  • T3 is typically the total triiodothyronine level (ng/dL; sometimes reported as nmol/L).

What is TSH?

TSH, or thyroid-stimulating hormone, is a hormone produced by the pituitary gland in the brain. It plays a crucial role in regulating the function of the thyroid gland, which is responsible for controlling the body’s metabolism. A TSH test is often used to diagnose thyroid disorders, such as hypothyroidism or hyperthyroidism, by measuring the level of TSH in the blood.

How to Calculate TSH (Estimate)?

The following steps outline how to estimate TSH using the equations above:

  1. For a mathematical demonstration, choose the FT4-based or Total T3-based cohort equation. Neither is a substitute for a measured result.
  2. If using FT4 in ng/dL, convert to pmol/L by multiplying by 12.87.
  3. Compute log10(TSH) using the chosen equation.
  4. Convert back to TSH by calculating TSH = 10log10(TSH).
  5. Compare the arithmetic with the calculator above. Do not use the cohort output as your measured hormone level, a diagnosis, a treatment target or a medication instruction.

Example Problem:

Use the following variables as an example problem to test your knowledge:

FT4 (Free T4) = 1.2 ng/dL

T3 (Total T3) = 150 ng/dL

Using the educational FT4 cohort equation: FT4 = 1.2 × 12.87 = 15.444 pmol/L, so log10(model TSH) = 1.50 − 0.059 × 15.444 = 0.588804 and model TSH = 100.588804 ≈ 3.879752 mIU/L. This is a mathematical example, not a predicted individual laboratory result.