How does it tell primary from secondary disease?

In primary thyroid disease the TSH moves opposite to the thyroid hormones — a high TSH with low free T4 is primary hypothyroidism. When TSH and free T4 move in the same direction (both low, or an inappropriately normal TSH with low free T4), the tool flags a possible central or secondary picture for pituitary review.

What is subclinical disease here?

Subclinical hypothyroidism is a raised TSH with a normal free T4, and subclinical hyperthyroidism is a suppressed TSH with normal free T4 and free T3. The tool reports these distinctly because management differs from overt disease and often depends on the degree of TSH change and symptoms.

What is the non-thyroidal illness pattern?

Acutely unwell patients can show a low free T3, sometimes low free T4, with a normal or low TSH — the sick euthyroid or non-thyroidal illness pattern. The tool suggests it when free T3 is disproportionately low without a TSH rise, and recommends repeating tests after recovery rather than treating.

Do I have to enter free T3?

No. TSH and free T4 alone classify most patterns. Free T3 is optional and mainly helps confirm T3-predominant hyperthyroidism or a non-thyroidal illness picture, so leave it blank if it was not measured.

Can I rely on this for diagnosis?

No. It applies the standard interpretive grid to your numbers but cannot account for medications, pregnancy, assay interference, recent illness, or pituitary disease. Use it as a prompt and confirm with clinical assessment and repeat or specialist testing where needed.

Thyroid Function Result Interpreter

Thyroid blood tests are interpreted as a pattern, not as single numbers. This tool takes your lab’s reference ranges and the patient’s TSH, free T4, and optional free T3, then maps them onto the standard interpretive grid to suggest the most likely thyroid state.

How it works

Each value is first classified as low, normal, or high against the range you enter. The combination then selects a pattern:

TSH high  + fT4 low     -> primary hypothyroidism
TSH high  + fT4 normal  -> subclinical hypothyroidism
TSH low   + (fT4/fT3) high -> primary hyperthyroidism
TSH low   + fT4 & fT3 normal -> subclinical hyperthyroidism
TSH low/normal + fT4 low -> possible central (secondary) hypothyroidism
fT3 low, TSH not raised, unwell -> non-thyroidal illness pattern
all normal -> euthyroid

Because TSH is exquisitely sensitive, the direction it moves relative to the thyroid hormones is what separates primary, central, and subclinical pictures.

Example and notes

A TSH of 8.5 mU/L (range 0.4–4.0) with a free T4 of 9 pmol/L (range 12–22) reads as primary hypothyroidism: the pituitary is driving hard but the gland is underperforming. The same high TSH with a normal free T4 would instead read as subclinical. Always interpret alongside symptoms, thyroid antibodies, drug history, and pregnancy status, and repeat borderline results before acting.