Goitre  

Fig.1: Large multinodular goitre, mainly affecting the left side of the thyroidAn enlarged thyroid is known as a goitre, derived from the Latin word guttur, meaning throat. It is actually just a descriptive term rather than a medical condition (Fig. 1).

This enlargement can be diffuse and affect the whole thyroid gland, or only affect part or all of one lobe. Goitres usually are not overactive and continue to produce the normal amount of thyroid hormone, even when they have grown to a very large size or become entirely nodular.

Some goitres however can have active nodules within them that overproduce thyroid hormone and cause thyrotoxicosis (overactive thyroid).

Lumps or nodules in the thyroid can be single or multiple. When there are many lumps in the thyroid it is called a multinodular goitre, which is the most common form of thyroid disease in Australia.

It is important to note that the vast majority of thyroid lumps are not cancerous. Further information about single thyroid nodules and how they are investigated can be found on the webpage Thyroid nodules.

Causes of goitre

Goitrogens

Certain substances called goitrogens can induce the formation of goitres. These substances are usually ions that interfere with iodide uptake by the thyroid and cause a decrease in thyroid hormone production. This results in the thyroid being stimulated by TSH, because of the lack of negative feedback (see webpage on thyroid Function). The TSH stimulation causes the gland to grow in size.

Examples of goitrogens include certain members of the brassica vegetable family, like cabbage, broccoli and cauliflower, and certain drugs and chemicals, like thiocyanates, phenytoin, carbamazepine, lithium and amiodarone.

Iodine deficiency

Iodine is needed by the thyroid gland to make thyroid hormones. The daily requirement for iodine is actually very small at about 150 mcg (microgram or millionths of a gram). This means that a person needs only a single teaspoonful over their whole life. The iodine in the diet is normally obtained from food and water, but there are some areas where the soils and drinking water are low in iodine, resulting in thyroid disease, especially hypothyroidism and goitre (from overstimulation by TSH). 

The iodine deficient regions of the world are generally in elevated and alpine areas far from the sea. Many parts of SE Asia are iodine deficient, as is Switzerland and parts of the UK (the 'Derbyshire neck').

In Australia the iodine deficient areas follow the Great Dividing Range, including New England, Canberra, Gippsland (the 'Gippsland Goitre') and Tasmania. Iodine deficiency was reduced in Australia through iodine supplementation in salt and bread as a public health measure. The use of iodine-based disinfectants in the dairy industry also led to increased iodine in milk.

There is some evidence that iodine deficiency has re-emerged as a health concern in recent years in Australia as levels of salt have been reduced in the diet, and the dairy industry has changed its practices. Health authorities and the food industry are looking at ways to increase the amount of iodine in the diet.

Iodine deficiency is a very serious problem for newborns and young children because thyroid hormones are essential for growth and the normal development of the brain. Inadequate dietary iodine leads to stunted growth and significant intellectual impairment - which is referred to as cretinism.

The recommended treatment is iodine supplementation in the diet, using iodised salt tablets or foods rich in iodine, particularly seafoods, including seaweeds.

This needs to be done carefully, with adequate monitoring with urinary iodine concentrations, etc., as iodine supplementation can worsen certain pre-existing thyroid diseases. The normal thyroid gland is tolerant of substantial excess iodine, but a diseased thyroid is not. A person with goitre or Graves' disease in remission may be precipitated into thyrotoxicosis, and the goitre can grow in size as well from excess iodine exposure. Some supplements like kelp tablets can contain milligram amounts of iodine and deliver hundreds of times the daily requirement, hence the need to carefully monitor iodine intake.

In contrast, large quantities of iodide have an inhibitory effect on the thyroid, and can be used to decrease the activity of a highly toxic gland for a limited time (such as with Lugol's iodine), but the mechanism is not fully understood.

Thyroid tumours

Goitres can also be caused by benign or malignant tumours of the thyroid. It is important to say again that the vast majority of goitres are not cancerous, and are related to benign causes from nodular disease.

Clinical Features of Goitre

Goitre is an enlargement of the thyroid, usually with no cancer or inflammation and normal thyroid function. It is the result of TSH stimulation, which causes diffuse enlargement of the thyroid, although with time it tends to become more nodular (lumpy).

The symptoms of a goitre can be related to either the physical size of the gland, a change in the function of the thyroid, or the development of symptoms related to a cancer forming inside the goitre.

As the goitre gets bigger, it may cause the following problems, as various structures in the neck are pressed on by the enlarging gland:

• Difficulty in swallowing
• Change in voice
• Breathlessness
• Facial congestion

Many patients leave their unsightly goitres for years, until they grow very large, or grow into the chest. They can develop worsening symptoms and complications when the goitre presses on the windpipe, the oesophagus, veins or nerves (Fig. 1).

As the goitre enlarges it can creep around the back of the windpipe (trachea) and press on the gullet (oesophagus), giving the sensation of a lump when swallowing. This is a very common symptom, and the cause can be confirmed by a CT scan of the thyroid showing the encroachment (Fig. 2).

Fig.2: CT of a goitre surrounding the windpipe, showing the posterior extension of the thyroid encroaching on the side of the oesophagus lying behind (arrow), causing the sensation of a lump when swallowing.

Once this process of pressure on neck structures starts, there is nothing to stop it continuing and the symptoms becoming worse over time.

This means there is little alternative for relief of symptoms, except removal of the encroaching gland at an operation.

Any patient with a goitre needs to be reviewed on a regular basis, and seek medical advice if any of the above symptoms develop.

Follow up is also important to make sure that a cancer doesn't develop in a long-standing goitre, which can occur on occasions. This would generally present as a change in a pre-existing goitre, with the enlargement of a single nodule, which becomes known as a dominant nodule. In this situation, a needle biopsy is the safest option to exclude malignancy.

Goitres may also lead to a change in thyroid function, becoming overactive or underactive (usually overactivity in practice), which can produce a variety of symptoms:

Medical treatment of a goitre

Thyroxine treatment has been used for years to try and stop goitres from growing, but the results are equivocal. The principle of this treatment is that thyroxine will decrease the TSH by negative feedback, but there is no evidence that thyroxine treatment changes the progression of a goitre. Small goitres do seem to be more responsive to thyroxine therapy than large ones, but once the treatment is stopped the goitre returns to its pre-treatment size.

In addition, thyroxine treatment has a number of serious potential problems, as small cancers can shrink initially on thyroxine treatment only to enlarge later, and there is a risk of making osteoporosis worse in postmenopausal women who take supplementary thyroxine in the hope it will control their goitres.

Indications for surgical treatment of a goitre

• Pressure signs or symptoms
• Growth down into chest (Retrosternal goitre)
• Suspicion of cancer on needle test or clinical examination
• Toxicity develops
• Cosmetic (patient's choice)

Surgical options

The best treatment for simple goitre is to remove all the thyroid tissue (total thyroidectomy) and this is now the favoured approach in most endocrine surgery units (see Thyroid Surgery). This is because it fixes the problem once and for all at a single operation, and can be done with excellent results and a low complication rate in specialist hands. By removing all of the thyroid tissue recurrence of the goitre is prevented, and if a cancerous nodule is found inadvertently in the specimen, usually no further surgery is needed.

Removing the nodules alone is usually unacceptable as it has a very high recurrence rate.

Subtotal thyroidectomy was performed in the past supposedly in order to prevent damage to other structures, but there is a 15% risk of recurrence of the goitre, and complication rates are no lower. If a second operation is required then the complication rate tends to be much higher because of scarring, so it is best to deal with the problem completely at the first operation.

The disadvantage of total thyroidectomy is the need for life-long thyroxine replacement, but this is easily managed and requires minimal monitoring by the specialist or GP.