What is free thyroxine (FT4)?
Thyroxine (T4) is the main hormone produced by your thyroid gland. Most of the T4 in your blood is bound to protein — meaning it's inactive and your body can't use it. A small amount is unbound (free). So by measuring your free T4 levels, it gives you a more accurate picture of your thyroid function.
What is its function?
Your body converts T4 to another hormone called triiodothyronine (T3). T3 helps control your metabolism — like regulating your heart rate, body temperature, and breathing.
What can cause it to change?
If your thyroid produces too much or too little T4, it can lead to hypothyroidism (an underactive thyroid) or hyperthyroidism (an overactive thyroid).
Low T4 levels might be caused by:
- autoimmune thyroiditis — your body produces antibodies that attack your thyroid gland, like Hashimoto’s disease
- treatment for an overactive thyroid
- treatment for thyroid cancer
- postpartum thyroiditis — your thyroid becomes inflamed within the first year after pregnancy
- iodine deficiency
- medications containing iodine e.g. amiodarone
High T4 levels might be caused by:
- Graves’ disease — an autoimmune disorder
- lumps (nodules) on your thyroid
- goitre — swelling of your thyroid gland
- postpartum thyroiditis
- medications containing iodine
Your FT4 levels can increase two hours after taking levothyroxine (a medication to treat an underactive thyroid). Or if you're over or under-treating an existing thyroid condition with medication, your levels could be out of range. Biotin (vitamin B7) might also affect your levels. It's best to take your blood sample first thing in the morning in a fasting state.
What are the most common symptoms?
If your thyroid is underactive, common symptoms include:
- weight gain
- dry skin
- brittle hair
- sensitivity to cold
- memory problems
- muscle aches
- hoarse voice
If your thyroid is overactive, common symptoms include:
- weight loss
- increased appetite
- excessive sweating
- sensitivity to heat
- irritability and anxiety
- heart palpitations
- difficulty sleeping
In women, high or low thyroid hormones might affect your periods — for example, heavy or irregular periods.
What can I do to change my levels?
There are a number of nutrients that are important for your thyroid function:
- Iodine — needed to make thyroid hormones
- Zinc — needed to make thyroid hormones
- Selenium — needed to thyroid hormones
- Iron — plays an important role in thyroid hormone metabolism
- Vitamin D — plays an important role in your immune function and low levels are linked to autoimmune thyroid disorders
If your levels indicate you have a thyroid disorder, it’s important to work with your doctor to come up with a treatment plan.