Free T4: What This Hormone Is and What the Test Shows

A referral for "free T4" often causes alarm: the name is there, but what it means is unclear. Let's sort this out without mystique: what this hormone is, why it is measured, and why a single number is almost never read on its own.

What T4 is and what the hormone is responsible for

T4, also known as thyroxine, also known as tetraiodothyronine, is the main form of thyroid hormones. The chemistry is encoded in the name: the molecule contains four iodine atoms. It is produced by the follicular cells of the gland, and the command comes from the pituitary hormone thyrotropin — TSH. When there is little T4 in the blood, inhibition is lifted, TSH rises and spurs the gland on; when there is a lot of T4, the production of TSH and thyroliberin is suppressed.

Thyroxine itself is biologically largely inactive; it is a prohormone. In tissues it is converted into triiodothyronine — T3 — and that is what does the work. It is T4 that enters the blood: from two-thirds to four-fifths of all thyroid hormones. It acts for a long time, so the body needs a steady level.

Free and total T4: what the difference is

In the blood, thyroxine is transported by proteins: thyroxine-binding globulin, transthyretin and albumin. A small portion of thyroxine circulates free, while the main part is bound to proteins. Free T4 is precisely the unbound fraction.

Hence two different tests. Total T4 shows everything together, while free T4 shows only the unbound fraction. When assessing how the gland is working, free T4 is looked at together with TSH.

When a free T4 test is ordered

Most often — when reduced thyroid function is suspected. The reason is symptoms: pronounced fatigue, feeling cold, weight gain, constipation, difficulty concentrating, low mood, dry skin and hair loss, a hoarse voice, irregular periods. Symptoms build up slowly and are easily put down to tiredness.

The doctor will ask about a family history of thyroid and autoimmune diseases, about a recent pregnancy, and will examine the neck — to see whether the gland is enlarged. Next comes a blood test for TSH and T4, and, if necessary, a referral to a specialist.

What the test shows and how to read the result

Free T4 shows the concentration of the unbound fraction of thyroxine. A diagnosis is not made from a single number: the result is assessed together with TSH and symptoms, and it is interpreted by a doctor.

High and low T4: what this may indicate

An excess of thyroxine is hyperthyroidism; its extreme degree, Graves' disease, can lead to heart failure. A deficiency is hypothyroidism: at an early age it can lead to cretinism, and at a more mature age — to myxedema.

One of the main causes is Hashimoto's disease, when the immune system attacks the body's own gland (more often in women, usually at 30–50 years of age). There are other thyroid diseases, surgery and radiotherapy to the neck, treatment for hyperthyroidism, a lack of iodine in the diet, and the use of certain medications. A family history also increases the likelihood.

Why the lack of treatment is dangerous: there is a higher risk of heart problems, including coronary heart disease and heart failure, and of bone problems, including osteoporosis. In very rare cases, severe hypothyroidism leads to myxedematous coma — a life-threatening condition treated in hospital. During pregnancy, untreated hypothyroidism increases the risk of preeclampsia, birth defects, premature birth and miscarriage.

When to see an endocrinologist

First, you can see a general practitioner or family doctor: they will order tests and, if necessary, refer you to a specialist. Pregnant women and those planning pregnancy with thyroid problems need additional examinations and monitoring.

• fatigue and feeling cold that are not explained by your routine and do not go away for months;
• weight gain without changes in diet, constipation, dry skin and hair loss;
• low mood, difficulty concentrating, a hoarse voice;
• irregular or heavy periods;
• pregnancy or planning pregnancy with already known thyroid problems.

A free T4 result on its own is not a reason to call an ambulance. 103 or 112 — if signs of a heart attack or stroke appear, or if a person with severe hypothyroidism suddenly deteriorates.

A number without context is just a number. This material is for reference and does not replace a doctor's consultation.

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