T4 (thyroxine) test explained

Thyroxine (T4) is the main hormone produced by the thyroid gland, and it controls how quickly the body uses energy — the heart rhythm, the metabolic rate, body temperature, digestion and even mood. In the blood, T4 travels mostly bound to proteins (total T4 covers both the bound and the free form), and only the small free fraction is biologically active — that part is measured by a separate free T4 (FT4) test.

T4 production is controlled by the pituitary gland, which releases TSH (thyroid-stimulating hormone): when there is little T4, TSH rises to push the thyroid, and when there is a lot of T4, TSH falls. That is why T4 is almost always assessed together with TSH, and often with T3 as well — together they make it possible to tell whether the problem is in the thyroid gland itself or in the pituitary gland. A total T4 result can be affected by pregnancy, oral contraceptives and some liver or kidney conditions, because these change the amount of the proteins that T4 binds to — which is why many laboratories today prefer the free T4 test.

• When a TSH result is abnormal and it needs to be clarified whether thyroid function is impaired
• With signs of hypothyroidism — tiredness, weight gain, intolerance of cold, dry skin, constipation
• With signs of hyperthyroidism — weight loss, a fast heartbeat, nervousness, sweating, tremor
• When a pituitary disorder, thyroid nodules or a goitre is suspected
• With difficulty conceiving or during pregnancy, when thyroid function needs to be assessed
• To monitor the course of treatment of an already diagnosed thyroid disease

The normal range for total T4 depends on the laboratory's method and on the reagents it uses, so you will find the interval that applies to you on your own report, next to your result. MedlinePlus states that the result has to be assessed together with TSH and the other thyroid tests, and not on its own.

A raised T4

According to MedlinePlus, a raised T4 can indicate hyperthyroidism, Graves' disease, certain stages of thyroiditis, a toxic goitre or a toxic thyroid nodule, too much iodine, a benign pituitary tumour, or too high a dose of thyroid hormone. The result is always assessed together with TSH and the other thyroid tests, not on its own.

• Hyperthyroidism (an overactive thyroid)
• Graves' disease
• Certain stages of thyroiditis (inflammation of the thyroid)
• A toxic goitre or a toxic thyroid nodule
• Too much iodine
• A benign pituitary tumour
• Too high a prescribed dose of thyroid hormone

What is usually looked at with a raised T4

• The TSH and free T4 (FT4) results together — they help to pinpoint the source of the disorder more precisely
• Symptoms of hyperthyroidism — palpitations, weight loss, tremor, sweating
• Whether thyroid hormone preparations are being taken, and at what dose
• Whether further thyroid imaging (an ultrasound scan) or an antibody test is needed

According to MedlinePlus, a low T4 can indicate hypothyroidism, Hashimoto's disease, certain stages of thyroiditis, a pituitary disorder, or too much or too little iodine in the diet. In newborns, a low T4 can indicate congenital hypothyroidism, which is why it is checked during newborn screening.

• Hypothyroidism (an underactive thyroid)
• Hashimoto's disease
• Certain stages of thyroiditis (inflammation of the thyroid)
• A pituitary disorder
• Too much or too little iodine in the diet
• Congenital hypothyroidism (in newborns)

What is usually looked at with a low T4

• The TSH result together with T4 — it helps to tell apart problems arising in the thyroid gland from those arising in the pituitary gland
• Symptoms of hypothyroidism — tiredness, weight gain, intolerance of cold
• The amount of iodine in the diet
• In newborns — the screening results and further monitoring

Related pages

Related pages on this site: TSH, free T4 (FT4), T3, free T3 (FT3), TPO antibodies (anti-TPO), thyroglobulin antibodies (anti-Tg), which thyroid tests you need, how to prepare for thyroid tests and how to read blood test results.