For educational purposes only.
The thyroid is a soft, butterfly-shaped gland that lies wrapped around the windpipe below the Adam's apple. Its job is to secrete thyroid hormones that help regulate metabolism - in other words, the many chemical processes that keep our bodies ticking over.
The thyroid gland manufactures two essential hormones – thyroxine (also referred to as T4) and triiodothyronine (also referred to as T3). In normal health most T3 circulating in the blood stream has been converted from T4. Both T4 and T3 have the same action on cells of the body but T3 is more potent. T4 mostly functions as a reserve for T3. Together these two similar hormones are called thyroid hormone.
Thyroid hormone regulates the metabolic rate of almost all the cells of the body, and influences the health of the heart, brain and bones. It is also needed for normal development of the brain in children and for normal reproductive functioning.
In Australia there are about 60,000 new cases of thyroid disease each year. Some people have a problem with how the gland functions: it may produce too much or too little thyroid hormone. Others have problems with the structure of the gland itself, where it becomes enlarged, lumpy, and/or inflamed.
Healthy thyroid activity
When the gland is healthy, it releases as much thyroid hormone as we need to keep our metabolism on an even keel. It knows just how much to release because of the feedback loop between a gland in the base of the brain, called the pituitary, and the thyroid itself. To function normally, the pituitary also requires a hormonal signal from a small part of the brain above the pituitary, called the hypothalamus.
The pituitary contains special cells that are very sensitive to levels of thyroid hormone in the blood. If the level is too low, the pituitary secretes a hormone called thyroid stimulating hormone (TSH) which stimulates the thyroid to produce more thyroid hormone.
When the level of thyroid hormone in the blood goes back to normal, the pituitary stops producing extra TSH.
If there's too much thyroid hormone in the blood: the pituitary releases less TSH, and the thyroid makes less thyroid hormone. This whole process is normally smoothly regulated by tiny pulses of TSH; allowing our bodies to maintain normal levels of T4 and T3, and a normal TSH level.
The hypothalamus has a hand in this too. It stimulates the pituitary to produce TSH by secreting thyrotropin releasing hormone (TRH). Tiny pulses of TRH are sent out cyclically throughout the day.
The whole system is called the hypothalamic -pituitary -thyroid axis.
The importance of iodine
The thyroid makes thyroid hormone from an amino acid called tyrosine that we get from proteins in our diet. But it also requires a regular supply of iodine from our diets. Adults need about 150 µg (micrograms) each day and 200 – 250 µg during pregnancy and breast-feeding.
Iodine is found in most foods and is especially prevalent in seafood and dairy products. It's often added to foods in the form of iodised salt too. However, some mountainous areas have soil that is low in iodine. As a result, foods grown there are iodine deficient.
Iodine deficiency was considered to be rare in Australia, occurring only in areas like Tasmania and along the Great Dividing Range where the soils are low in iodine. However mild iodine deficiency has re-emerged as a health concern in recent years particularly in New South Wales, Victoria and Tasmania, and especially in pregnancy.
In response, legislation was passed in 2008 requiring that salt used in commercially-baked bread in Australia and New Zealand must be iodised. The only exception is organic bread. The WHO advocates universal salt iodisation but this policy has not been adopted yet in Australia.
Hyperthyroidism is when the thyroid becomes overactive and releases too much T3 and T4 into the blood. Another name for it is thyrotoxicosis. If you have too much circulating thyroid hormone your metabolism is continually 'revved up' and you may notice the following symptoms:
- weight loss, despite a normal or increased appetite
- nervousness, agitation and anxiety
- physical weakness (muscular)
- rapid pulse
- tremor (shaking) of the hands
- sweating and sensitivity to heat
The condition can be caused by a variety of factors including viral infections, a single lump or 'nodule' in the thyroid, or multiple nodules (see below).
However, the most common cause of hyperthyroidism is a condition called Graves' disease.
Graves' disease is one of a group of disorders called autoimmune diseases in which the body's antibodies turn and attack the body's organs.
In the case of Graves' disease, the attacking antibodies overstimulate the thyroid causing it to make excessive amounts of thyroid hormone.
Graves' disease usually begins in young to mid-adult life but can rarely occur in children and teenagers, and in the elderly. It's at least six times more common in women than men, and it may occur with other autoimmune diseases like diabetes and rheumatoid arthritis. The exact cause of the condition is unknown but genetic factors clearly predispose some people to the condition. It is more common and more severe in smokers.
People with Graves' usually experience some or all the symptoms of hyperthyroidism described above.
In addition, high thyroid hormone levels can lead to the upper eye lids retracting, resulting in a staring expression. About half of people with Graves' will also have protuberant eyeballs (called exophthalmos or proptosis). This condition happens when the muscles and fat inside the eye cavity enlarge, pushing the eye forward. This is another manifestation of autoimmunity, and is more frequent and more severe in smokers.Less than 5% of people with Graves' disease get this severely but it can be a distressing problem even in a milder form. Ophthalmologists with expertise in this condition can often provide helpful treatment.
In hypothyroidism, the thyroid fails to make and release enough T3 and T4 into the bloodstream and if severe, the metabolism slows to a crawl. This produces a range of physical and emotional changes, including:
- lethargy and tiredness
- feeling cold (even on warm days)
- difficulty concentrating
- unusual weight gain
- puffiness of the face
- hair loss
- dry skin
Hypothyroidism can be very mild and without symptoms, but on the other end of the scale, people can experience full-blown symptoms as described above. A blood test will reveal subtle changes in blood levels of T3 and T4 even if you have very mild thyroid failure, so a blood test by a GP is crucial for proper assessment.
Hypothyroidism usually happens because the thyroid itself fails (primary hypothyroidism).
But it can also occur when the thyroid gland is healthy but the pituitary or hypothalamus fail to secrete enough hormone to keep the 'hypothalamic-pituitary-thyroid-axis' working adequately.
This situation, called secondary or central hypothyroidism, is less common than the primary type and is usually the result of damage to the pituitary from a tumour, or surgery. Damage to the hypothalamus above the pituitary can also result in hypothyroidism.
The most common cause in Australia of primary hypothyroidism, where the thyroid itself fails, is a condition called Hashimoto's disease, or Hashimoto's thyroiditis. Like Graves' disease, Hashimoto's is an autoimmune disease, where the body's antibodies attack the thyroid gland. It can also occur with other autoimmune diseases like rheumatoid arthritis or diabetes. It is more common in women than men, and it usually starts after the age of 30.
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