TSH, T3 and T4: What Thyroid Hormone Tests Show

The thyroid gland is a small butterfly-shaped organ in the front of the neck that produces hormones controlling how the body uses energy. These hormones affect almost every organ: breathing, heart rate, weight, digestion and mood. Blood tests show how well the gland is doing its job.

TSH, T3 and T4: what these hormones are

TSH (thyroid-stimulating hormone) is produced by the anterior pituitary gland. Structurally it is a glycoprotein made of two subunits, and its receptors are located on the surface of thyroid gland cells. T3 (triiodothyronine) and T4 (thyroxine) are hormones of the gland itself; TSH stimulates their production and activation.

What TSH is responsible for and how it relates to T3 and T4

TSH secretion is regulated by a negative feedback system. TSH has daily fluctuations: the maximum occurs at 2–4 a.m., the minimum at 5–7 p.m. During pregnancy the TSH concentration decreases, and with age it becomes slightly higher.

Free T4 and TSH: how they are related

T3 and T4 tests measure the levels of different thyroid hormones in the blood. There is a separate test for free T4. This is why TSH and T4 are often assessed together. The doctor decides which specific test to order.

Which thyroid tests are ordered most often

When insufficient gland function is suspected, the doctor asks about symptoms, family cases of thyroid and autoimmune diseases, recent pregnancy, and may examine the neck. Then blood tests for TSH and T4 are ordered. TSH usually becomes the first test the doctor orders. Additionally there may be a thyroid antibody test, and among imaging methods — ultrasound, CT, thyroid scanning and a radioactive iodine uptake test.

• TSH — usually the first test when gland dysfunction is suspected
• T3 and T4 — levels of different thyroid hormones in the blood
• Thyroid antibodies — help when an autoimmune disease is suspected
• Ultrasound and scanning — assess the size, shape and position of the gland

What changes in the values may mean

Abnormal test results alone do not establish a diagnosis: they are assessed together with complaints and examination. With insufficient gland function, fatigue, feeling cold, weight gain, constipation, difficulty concentrating, low mood, dry skin and hair, hoarse voice, irregular or heavy periods are possible. Symptoms develop slowly. One of the main causes of insufficient gland function is autoimmune Hashimoto's thyroiditis, more often in women, usually at the age of 30–50.

When to see an endocrinologist

You should see a doctor if the listed complaints appear: some of them also occur in other conditions, but they need to be checked. During pregnancy or when planning pregnancy, tell the doctor about insufficient gland function: additional tests will be needed. If insufficient gland function is not treated, the risk of heart and bone problems increases.

Abnormal hormone values alone are not a reason to call an ambulance; 103/112 — if myxedema coma is suspected (a rare severe complication of thyroid insufficiency, more often in older people) — this is an emergency condition requiring hospital treatment.

This material is for reference: a diagnosis is not made from test numbers alone, so discuss the results with your doctor.

This content is for reference and does not replace a doctor's consultation. Medical disclaimer