Pertechnetate thyroid scan

Brief introduction

Principle

Normal function of thyroid

Thyroid is a small organ in the anterior part of lower neck. Thyroid cells called 'thyrocytes' produce thyroid hormones called 'T3' (triiodothyronine) and 'T4' (thyroxine), which are released into the blood to reach all the organs of the body. A small fraction of cells within the thyroid also produces a hormone called 'calcitonin', which is not relevant to the present discussion.

The thyroid hormones are made incorporating the element 'iodine'. In fact, the names T3 and T4 are derived from the respective number of iodine atoms on each molecule of an amino acid called 'tyrosine'. Iodine from the blood is extracted by the thyrocytes using a protein called 'sodium-iodide symporter'.

Thyroid produces much more of T4 than T3, but latter is the more active form of the two. In fact, most of the T4 is converted to T3 outside the thyroid. Both thyroid hormones have effects on most of the organs of the body. They are also very vital for proper growth and development during childhood.

Effects of thyroid hormones vary from organ to organ, but in most simple terms they promote an 'active' state of the body.

Another hormone called 'thyroid stimulating hormone' (TSH) produced by the pituitary gland (located in lower part of the brain) is essential for formation and release of thyroid hormones by the thyroid. A feedback mechanism exists between thyroid hormones and TSH, i.e., TSH increases thyroid hormone levels in the blood, but thyroid hormones decrease the blood level of TSH. Conversely, low levels of thyroid hormones in the blood lead to high level of TSH in the blood, and high levels of thyroid hormones result in suppressed TSH blood level.

Altered function of thyroid

Thyroid could either overfunction ('hyperthyroidism') or could underfunction ('hypothyroidism'). A state of increased level of thyroid hormones in the blood or clinical features compatible with the same is known as 'thyrotoxicosis'. Thyrotoxicosis occurs most commonly because of hyperthyroidism (i.e., increased production of thyroid hormones), but occasionally can also be associated with a underfunctioning or normally functioning thyroid gland. Also, sometimes only small parts of the thyroid gland (called 'adenoma' - a type of benign tumour) show altered (increased or decreased) function. Very rarely, cancerous tumours could also be present within the thyroid, which show reduced function.

Symptoms associated with thyrotoxicosis can be highly varied depending upon levels of thyroid hormone levels, rate of increase of thyroid hormone levels, duration of thyrotoxicosis and age of the patient. Some of the common symptoms experienced by the patients include the following.

• Heat intolerance and excessive sweating.
• Palpitations (heightened awareness of heart beats).
• Weight loss.
• Weakness.
• Increased frequency of stools.
• Tremors (involuntary movements of small muscles, e.g., fingers).
• Increased anxiety.
• Increased appetite.

Some of the health problems associated with long-lasting uncontrolled / poorly controlled thyrotoxicosis include:

• Infertility.
• Difficulty in controlling blood glucose level.
• Increased strain on heart.
• Protein loss.
• Weakening of bones.

Symptoms of hypothyroidism tend to be roughly opposite to that of thyrotoxicosis, and most commonly include:

• Weight gain.
• Lethargy.
• Constipation.
• Cold intolerance.

A bit more about hyperthyroidism

Hyperthyroidism is a state wherein thyrocytes function more than needed (usually independent of stimulation by TSH) and produce unhealthily excess amounts of thyroid hormones, which in turn causes thyrotoxicosis (some people use the terms 'hyperthyroidism' and 'thyrotoxicosis' interchangeably). The TSH level in hyperthyroidism is usually low. 'Goitre' is a term used for enlarged thyroid; it may or may not be associated with abnormal blood thyroid hormone levels. Major causes of hyperthyroidism include:

• Graves' disease. In this condition proteins called 'antibodies' attach to certain types of body cells (most notably, thyroid, skin and tissue behind the eyes) causing swelling and malfunction. Thyrocytes 'mistake' these antibodies to be TSH, and in response grow in numbers and overproduce thyroid hormones. This is the most common cause for hyperthyroidism. As it is an autoimmune condition (i.e., the body's immune system wrongly attacks body's own cells), in some patients there are symptoms related to skin and eyes. Even more rarely there could be damage to other organs as well (e.g., stomach lining). These autoimmune effects on other organs are independent of excess blood levels of thyroid hormones. In some cases, hyperthyroidism can spontaneously transform into hypothyroidism.
• (Toxic) multinodular goiter (MNG). Multiple small nodules (bumps) are seen throughout the thyroid gland. These nodules tend to be very heterogeneous, i.e., ranging from being underfunctioning to overfunctioning, and hence, not all cases of MNG are associated with thyrotoxicosis. In fact, most of the patients start out with underfunctioning or normally functioning thyroid. Past iodine deficiency increases the risk of development of MNG. Overproduction of thyroid hormones is because receptors for TSH present on surface of thyrocytes remain abnormally active even without interaction with TSH. This abnormality in the TSH receptors is usually acquired and not present at birth. Thyrotoxicosis associated with MNG tends to be less severe compared to Graves' disease. However, blood TSH level tends to be low because of which normal thyrocytes actually function at lowered capacity.
• Autonomously functioning / solitary toxic thyroid nodule. The mechanism of thyrotoxicosis is similar to toxic MNG except that there is one or only few nodules. Consequently, thyroid hormone levels tend to be normal or only marginally raised, but blood TSH level tends to be lower than normal.

Hyperthyroidism is treated using one of the following strategies.

• Anti-thyroid drugs that suppress the functioning of thyroid resulting in reduced production of thyroid hormones.
• Radioiodine therapy, which involves administering radioactive form of iodine that gradually irreversibly destroys the thyrocytes.
• Surgical removal of thyroid gland.

Thyrotoxicosis without hyperthyroidism

While thyroid overfunction (hyperthyroidism) is the commonest cause of increased blood thyroid hormone levels, it is not the only cause. These levels can also be raised in case of sudden release of hormones from injury to thyrocytes or because of external sources of thyroid hormones (e.g., thyroxine tablets). Injury to thyrocytes can occur in following conditions.

• Physical injury like surgery or some kind of blunt trauma (very rare).
• Inflammation of thyroid because of infections or autoimmune response. This condition is called 'thyroiditis'. There can be other uncommon causes of thyroiditis as well (e.g., exposure to certain drugs).
• Radiation exposure to thyroid, e.g., as part of external beam radiotherapy for head-and-neck cancers or following radioiodine therapy (discussed elsewhere).

Indications (conditions in which advised)

• Distinguishing between hyperthyroidism and thyroiditis.
• To determine suitability for radioiodine therapy in suspected or established cases of hyperthyroidism.
• To determine functional versus non-functional nature of tumours of thyroid. This information in turn helps in estimating risk of cancer associated with such tumours. Please note: Nowadays, ultrasonography is the preferred mode of imaging for assessing cancer potential of a thyroid tumour, and pertechnetate thyroid scan has limited utility for this purpose.
• To look for functional thyroid tissue in those born with hypothyroidism.
• To look for ectopic thyroid tissue (thyroid at unusual location).

Please note: Radionuclide thyroid scan does not have a strong role in management of hypothyroidism.