Thyroid function assessment
The thyroid gland produces two main hormones — thyroxine (T4) and triiodothyronine (T3), and the pituitary's TSH governs them through a feedback loop. Enter your lab values — the calculator will show each hormone's status and a probable diagnosis, from euthyroidism to overt hypo- or hyperthyroidism.
Three hormones that set the tone
The thyroid gland produces thyroxine (T4) and triiodothyronine (T3) — thyroid hormones that govern a host of processes: from appetite and mood to cholesterol levels and heart rate. When they fall out of balance, symptoms appear that are easily mistaken for other conditions — depression or anemia.
T4 and T3 are controlled by thyroid-stimulating hormone (TSH), which the pituitary produces through a feedback mechanism: when hormones run low, TSH rises to spur the gland on. That is why it is the combination of TSH, free T4 and T3 that gives the full picture of how the gland is working.
Why look at all three, not just TSH
TSH is the most sensitive screening marker, but it reflects the gland's work only indirectly. Free T4 and T3 show how much active hormone is actually available to the tissues. Combining the three values makes it possible to tell apart closely related states — for example, subclinical and overt hypothyroidism — and to spot rare variants such as secondary (pituitary) dysfunction.
If you don't have lab results yet, start with the symptom screening — it will suggest which tests to order. And to assess not the hormones themselves but the gland's production reserve, there is a separate SPINA-GT secretory capacity calculation.
It is also important to understand the difference between the hormones. T4 is the "reserve": the gland releases it into the blood in large amounts, but on its own it is barely active. The real work in the cells is done by T3, and up to 80% of active T3 is formed not in the gland but in the tissues — the liver, kidneys and muscles — by removing a single iodine atom from T4. So a normal T4 with a low T3 sometimes points not to the gland itself but to a problem with this conversion.
How the feedback loop works
The gland is controlled like a thermostat. The hypothalamus releases TRH, which makes the pituitary produce TSH, and TSH stimulates the thyroid to make T4 and T3. As hormone levels in the blood rise, they suppress TSH production, and vice versa. This feedback loop is what explains the logic of the tests:
- TSH rises when there aren't enough hormones: the pituitary "shouts louder", trying to rouse a sluggish gland. That is why in hypothyroidism TSH climbs before T4 falls.
- TSH falls when there are too many hormones: the pituitary dials down its stimulation. A low TSH is a typical sign of hyperthyroidism.
- The exception is secondary dysfunction: if the pituitary itself is affected, TSH can be low alongside a low T4 — the loop is broken at the top level.
Thyroid function calculator
Enter the values from your blood test. TSH and free T4 are required, free T3 is optional (it refines the result). Check off any symptoms from the past 2 months. The calculation runs in your browser using a transparent model.
Symptoms over the past 2 months (context — they don't affect the diagnosis)
Hormone status
How to read the result
The probable diagnosis is determined by the combination of the three hormones. Here are the main patterns:
| TSH | Free T4 | Free T3 | Interpretation |
|---|---|---|---|
| High | Low | Low/normal | Hypothyroidism |
| High | Normal | Normal | Subclinical hypothyroidism |
| Low | High | High | Hyperthyroidism |
| Low | Normal | Normal | Subclinical hyperthyroidism |
| Low | Low | Low/normal | Secondary hypothyroidism |
| Normal | Normal | Normal | Euthyroidism (normal) |
Six possible results
The calculator reduces the hormone combination to one of six states. Here is what stands behind each:
Symptoms: slowing down and speeding up
Thyroid hormones set the pace of metabolism, so its disorders are mirror images: in hypothyroidism everything slows down, in hyperthyroidism it speeds up. The numbers in a test take on meaning alongside the symptoms:
| System | Hypothyroidism (slowing) | Hyperthyroidism (speeding up) |
|---|---|---|
| Temperature and weight | Feeling cold, weight gain | Feeling hot, sweating, weight loss |
| Heart | Slowed pulse | Palpitations, skipped beats |
| Mind and sleep | Sluggishness, "brain fog", depression | Anxiety, tremor, insomnia |
| Digestion | Constipation | More frequent stools |
| Skin and hair | Dryness, hair loss | Thin, moist skin, brittleness |
| Energy | Chronic fatigue | Agitation, muscle weakness |
Optimal ranges, not just reference ones
The laboratory "normal" range is wide, and pronounced symptoms with formally normal numbers are not uncommon. It helps to aim for optimal windows: TSH 0.5–2.5 mIU/L, free T4 1.4–1.7 ng/dL, free T3 3.8–4.4 pg/mL. A TSH above 2.5 mIU/L is already considered an early signal: the pituitary is working harder than usual to stimulate the gland.
What can distort the result
Thyroid hormones are a sensitive marker, and a single test is influenced by many things. Before drawing conclusions, keep in mind:
- Time of day and how it's taken. TSH is higher in the morning and on an empty stomach and falls toward evening, so it is better to compare tests taken under the same conditions.
- Acute illness and stress. A severe illness or surgery temporarily changes hormone levels ("euthyroid sick syndrome") — this is not a disease of the gland itself.
- Medications. Levothyroxine, corticosteroids, estrogens, amiodarone, lithium and even high doses of biotin (vitamin B7) can distort the result — it is best to stop biotin 2–3 days before the test.
- Pregnancy. TSH norms differ by trimester from the usual ones — a doctor interprets them separately.
- A single measurement is just a snapshot. Borderline values are almost always worth retesting before changing anything.
What to do with the result
- Confirmation. A single test is a snapshot. Borderline values are worth retesting in 6–12 weeks.
- Antibodies. With an elevated TSH, TPO antibodies are useful — to detect autoimmune thyroiditis.
- Nutrition. Enough iodine and selenium — the basic building blocks for hormone synthesis.
- A doctor. The decision about therapy is made by an endocrinologist based on the totality of the data, not on a single calculator.
The key takeaway: a three-hormone panel is more accurate than TSH alone and helps catch dysfunction early — but it remains a tool to support a doctor's decision, not a replacement for it.
Frequently asked questions
Can I diagnose myself with the calculator?
No. It is an interpretation of lab results that helps you understand the picture and prepare for a conversation with your doctor. The final diagnosis is made by an endocrinologist, taking into account antibodies, ultrasound, symptoms and the medications you take.
Why is TSH considered the main marker?
TSH responds to the slightest changes before the hormones themselves move outside the normal range: the pituitary strengthens or weakens its stimulation of the gland via feedback. That is why screening almost always starts with TSH.
Is it necessary to test free T3?
For a basic assessment, TSH and free T4 are enough. Free T3 refines the picture when hyperthyroidism is suspected and in ambiguous cases, which is why it is optional in the calculator.
What does "subclinical" mean?
It is an early stage, when TSH has already shifted while the free hormones are still normal and there may be no obvious symptoms. Such states respond well to monitoring and correction, so it is important not to miss them.
How often should the thyroid be checked?
With normal results and no complaints — about once a year, especially for women over 35 and those with a family history. With borderline values or symptoms, a doctor will schedule a follow-up in 6–12 weeks.
