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.

What is the Thyroid Function Result Interpreter?

Takes TSH, free T4, and optional free T3 with their reference ranges and classifies thyroid function — primary or secondary hypo/hyperthyroidism, subclinical states, and a non-thyroidal illness pattern. A teaching and triage aid for GPs and endocrinologists. It runs free in your browser on Gera Tools, with nothing uploaded.

Thyroid Function Result Interpreter

Name: Thyroid Function Result Interpreter
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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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.

Why TSH moves opposite to T4 in primary disease

The pituitary monitors circulating free T4 and T3 via negative feedback. When the thyroid gland underproduces T4, the pituitary detects the shortfall and increases TSH secretion in an attempt to stimulate the gland. The result is a high TSH with a low (or falling) T4 — the hallmark of primary hypothyroidism. The inverse occurs in primary hyperthyroidism: excess T4 and T3 suppress the pituitary, causing a very low or undetectable TSH.

In central (secondary or tertiary) disease, the problem lies in the pituitary or hypothalamus rather than the gland. TSH may be low or inappropriately normal even when T4 is low — the pituitary is not responding normally to the signal it would otherwise amplify. This breaks the expected inverse relationship.

Common patterns with worked examples

Example 1 — Primary hypothyroidism TSH 8.5 mU/L (range 0.4–4.0), free T4 9 pmol/L (range 12–22). Both out of range in opposite directions: TSH high, T4 low. The pituitary is compensating maximally but the gland cannot respond. This is overt primary hypothyroidism requiring treatment.

Example 2 — Subclinical hypothyroidism TSH 6.2 mU/L (range 0.4–4.0), free T4 14 pmol/L (range 12–22). TSH elevated but T4 still within range — the gland is still producing enough hormone but only under extra pituitary drive. Subclinical hypothyroidism. Management depends on the TSH level, symptoms, antibody status, and patient factors.

Example 3 — Subclinical hyperthyroidism TSH 0.05 mU/L (range 0.4–4.0), free T4 16 pmol/L (range 12–22), free T3 5.2 pmol/L (range 3.5–6.5). TSH suppressed but both hormones in normal range. The gland is beginning to run autonomously. Subclinical hyperthyroidism carries cardiac and bone risks that differ by degree of TSH suppression.

Example 4 — Non-thyroidal illness (sick euthyroid) TSH 1.8 mU/L (normal), free T4 12 pmol/L (low-normal), free T3 2.1 pmol/L (below range). An acutely unwell inpatient. The disproportionately low T3 with a normal TSH and borderline T4 suggests peripheral deiodination suppression rather than true thyroid disease. Repeat when recovered rather than treat.

When to enter free T3

Free T3 is optional because TSH and free T4 classify the majority of patterns. Enter free T3 when:

T3-predominant hyperthyroidism is suspected (suppressed TSH, normal T4, elevated T3)
Non-thyroidal illness pattern is possible in an unwell patient
Monitoring T3 replacement or combination therapy

This tool is a teaching and triage aid. It does not replace clinical assessment, and borderline results should be repeated before clinical decisions are made.