Triiodothyronine (T3-V)

The thyroid is a gland in the neck. It produces, stores, and excretes thyroid hormones, such as thyroxine (T4) and triiodothyronine (T3). Thyroid hormones have several significant functions, and they are needed, for example, for hormone production (oestrogen) and hormone activity (growth hormone), fat, glucose and bone metabolism, as well as bowel, gall bladder, and liver function.

Triiodothyronine is mainly present in the body bound to proteins, and less than about one per cent is transported free in the bloodstream (S-T3-V). Thyroxine (T4) has four iodine atoms attached, and when one iodine atom is released in other tissues, it becomes triiodothyronine T3 (tri Greek for three, thyreos Greek for shield).

When is a good time to measure the T3-V value?

T3-V, or free T3 hormone, can be measured when investigating hyperthyroidism, if necessary. It can reveal early hyperthyroidism in a situation in which the thyroid-stimulating hormone (TSH) is low, but free thyroxine (T4-V) is normal. In addition, T3-V should be tested in connection with the following symptoms, which could indicate hyperthyroidism:

• Trembling of the fingers
• Intolerance to cold
• Increased sweating
• Fatigue
• Nervousness
• Weight loss
• Increased appetite
• Atrial fibrillation
• Bulging eyes
• Swelling and dryness around the eyes
• Anxiety
• Sleep problems
• Weight loss
• Weakness

What does a T3-V test measure?

The T3-V test indicates hyperthyroidism. The level of free T3-V hormone is examined when suspecting hyperthyroidism or monitoring previously diagnosed hyperactivity.