Thyroxine (T4) is one of two hormones produced by the thyroid gland. It works to speed up the rate of your metabolism. Most T4 is bound to carrier proteins in the blood, but it is only the free, or unbound T4 that is active in the body, which is measured in this test.

Triiodothyronine (T3) is the more active of the two thyroid hormones produced by the thyroid gland. Most T3 is bound to protein in the blood. Free T3 measures the level of T3 that is free, or unbound to protein, and is available to regulate metabolism.

The thyroid is a gland at the base of your neck responsible for a number of metabolic processes, including energy expenditure, cardiac function, muscle physiology and substrate turnover. Disturbances in your thyroid function can lead to excess hormone levels (overactive) or diminished levels (underactive), both of which can lead to a decrease in athletic performance. Thyroid Stimulating Hormone (TSH) is produced by the pituitary gland and stimulates the thyroid gland to produce the two thyroid hormones thyroxine (T4) and triiodothyronine (T3). Thyroid hormone production is part of a neuroendocrine cascade. It starts in the hypothalamus with the release of thyrotropin releasing hormone (TRH), which triggers the pituitary gland to produce thyroid stimulating hormone (TSH). This binds to cells in the thyroid gland to release the hormones T3 and T4 (thyroxine). T4 is also converted into T3 (the more active thyroid hormone) at peripheral tissues. It is these hormones which essentially control the metabolism around your body. All these levels are normally held in tight balance through negative feedback loops. Abnormal thyroid function can manifest by over-secretion or under-secreting the thyroid hormones.

The thyroid is a butterfly shaped gland that sits under your Adam’s apple. It produces thyroid hormones which help to regulate your metabolism. Sometimes, the thyroid produces too many hormones, leading to an overactive thyroid (hyperthyroidism), or too few, leading to an underactive thyroid (hypothyroid).

As many as 1 in 20 people are thought to have a thyroid disorder. There are a number of different conditions that can affect the normal functioning of the thyroid, including an underactive thyroid (hypothyroid), an overactive thyroid (hyperthyroidism), autoimmune disease, thyroid swelling (or a goitre), nodules or thyroid cancer. Although anyone can develop a thyroid disorder, factors including being female or over the age of 50, and people with a family history of thyroid problems, can increase an individual’s risk of developing thyroid disease.

Test will depend on whether you have been diagnosed with a thyroid disorder and, importantly, whether you are experiencing symptoms. You may want to take a thyroid test if you are experiencing symptoms of an overactive or underactive thyroid or if you are managing a thyroid condition.

You can test your thyroid during pregnancy as it’s important to have a well-controlled thyroid for the healthy development of the baby. If you have a thyroid condition and are planning to conceive, or when find out you’re pregnant, let your GP know so they can monitor you or take your thyroid test results to your doctor.

Thyroid results during pregnancy:

• Underactive thyroid during pregnancy – TSH level should be less than 2.5 mU/l in the first trimester and less than 3.0 mU/l in the second and third trimesters.
• Overactive thyroid during pregnancy – doses may be adjusted through regular monitoring.