Thyroid gland and thyroid hormones
The thyroid gland is found in front of the trachea (wind pipe) in your neck. The gland is divided into 2 lobes (right and left) and is connected in the middle by a thin bridge of thyroid tissue, known as the isthmus. Because of the 2 connected lobes, the thyroid has been described as being shaped like a butterfly, or a bow tie.
The thyroid cartilage, which is the largest cartilage of the larynx (voice box) lies just above the thyroid gland and is sometimes known as the Adam's apple.
A normal-sized thyroid gland cannot be seen in the neck, and can barely be felt. It is only when certain conditions result in an enlarged thyroid gland (known as goitre), that a bulge may be seen or felt just underneath the Adam’s apple.
What is the function of the thyroid gland?
The thyroid gland is part of the endocrine system of the body. Organs of the endocrine system secrete hormones. The primary function of your thyroid gland is to secrete thyroid hormones. The thyroid hormones are involved in regulating many of your bodily functions, such as your breathing, heart rate, temperature, how quickly you burn calories, and digestion, among other functions. Babies and children need adequate amounts of thyroid hormones for brain development and growth.
Your thyroid needs iodine (a chemical element that’s an essential part of our diet) in order to manufacture these thyroid hormones. Foods that are naturally rich in iodine include seafood and plants grown in iodine-rich soil. Iodised salt is another good source of dietary iodine.
What hormones does the thyroid gland secrete?
The 2 main thyroid hormones are T3 (triiodothyronine) and T4 (thyroxine). T3 and T4 regulate your body's temperature, metabolism and heart rate.
The amount of thyroid hormones secreted is controlled by another hormone, called thyroid stimulating hormone (TSH), which is released from the pituitary gland in your brain. TSH stimulates the thyroid to make T3 and T4. Blood tests are done for TSH levels when doctors investigate for thyroid disease. T3 and T4 may also be tested for.
Note that different laboratories may have different reference ranges for normal, so always follow the advice of your doctor.
T3 or triiodothyronine
T3 tests can help diagnose an overactive thyroid (hyperthyroidism). The usual accepted normal range for free T3 (which measures T3 in the bloodstream, but not T3 bound to protein in your body) is between 3.1 pmol/L and 6.8 pmol/L.
The normal range usually quoted for free thyroxine (T4) is 12-22 pmol/L. In people with hyperthyroidism (overactive thyroid), free T4 is usually above 22 pmol/L. And for those with hypothyroidism (underactive thyroid), free T4 is usuallly below 12 pmol/L.
Symptoms of an underactive thyroid
Hypothyroidism, where the thyroid is underactive, can result in weight gain, tiredness, feeling cold, dry skin and frequent periods.
Symptoms of an overactive thyroid
Hyperthyroidism, where the thyroid is overactive, can result in symptoms such as weight loss, rapid heart rate, being too hot, feeling nervous or edgy, and irregular periods.
Another hormone that’s produced in your thyroid gland is called calcitonin. This hormone, secreted by a small population of cells known as C cells, is involved in regulating the level of calcium and phosphate in your blood.
The levels of calcitonin are driven by the amount of calcium in your blood. When your blood calcium levels decrease, less calcitonin is secreted and vice versa - when your blood calcium levels increase, levels of calcitonin increase. Calcium and phosphate are both involved in the formation of bones.
2. NHS Choices. Underactive thyroid (hypothyroidism). http://www.nhs.uk/Conditions/Thyroid-under-active/Pages/Introduction.aspx (accessed Feb 2016).
3. NHS Choices. Overactive thyroid. http://www.nhs.uk/Conditions/Thyroid-over-active/Pages/Introduction.aspx (accessed Feb 2016).