Diagnosis of Thyrotoxicosis

Fig. 1: Thyroid scan showing a toxic noduleThe diagnosis of thyrotoxicosis is usually straightforward, using a combination of clinical suspicion, blood tests and nuclear medicine scanning. The following tests should be performed:

1) Thyroid function tests (TSH, T4 and T3)

A low TSH will confirm the diagnosis of thyrotoxicosis due to hyperthyroidism, as long as the free thyroxine (T4) is raised. Low TSH levels will also be found in patients with pituitary failure, but in this case the T4 is also low.

Subclinical thyrotoxicosis is present when TSH is suppressed but T4 and T3 are normal, indicating incipient development of autonomous nodules and impending toxicity (often found in longstanding large goitres). On rare occasions the TSH is low and the T4 is normal, and only free triiodothyronine (T3) is secreted, in a condition called T3 toxicosis.

2) Thyroid autoantibodies

Antibodies to peroxidase, thyroglobulin and TSH receptors are found in 80-90% of cases of thyrotoxicosis due to Graves’ disease, but they can also be elevated in other thyroid conditions such as thyroiditis.

3) A Radioactive Iodine or Technetium Scan

Uptake of tracer doses of technetium or iodine is extremely useful (Fig. 1), and may show the following:

A low uptake or no uptake in the thyroid occurs in:

• Thyroiditis
• Post-partum thyroiditis
• Iodine induced thyrotoxicosis
• Factitious hyperthyroidism (self administered or over replacement of thyroid hormone)

A diffuse uniform uptake in the thyroid occurs in:

• Graves' disease

A patchy uptake in the thyroid occurs in:

• Toxic multinodular goitre

A single hot spot in the thyroid is seen in:

• Toxic adenoma

The thyroid scan is important to distinguish the various causes of thyrotoxicosis, as the treatment choices tend to be different for each diagnosis.

Treatment of Thyrotoxicosis

There are a variety of ways to treat thyrotoxicosis, which are detailed on separate webpages (just click on the name of the treatment below for more information). Each of the methods has advantages and disadvantages:

Drug treatment involves 9 – 12 months of therapy, with the risk of side effects, and even then there is a 50% chance that the thyrotoxicosis will flare up again as soon as the drugs are stopped, so it tends to be a poor long term solution.

Radioactive iodine is the best curative treatment for Graves’ disease, but may require several doses and necessitates long-term thyroid replacement. It also cannot be used during pregnancy, necessitates a 6-12 month delay before pregnancy is safe, and requires the patient to be excluded from contact with young children for a period after the therapy.

Thyroid surgery has the advantage of fixing the thyrotoxicity immediately after the operation, but has all the disadvantages of a surgical procedure, such as potential complications with the voice and parathyroids.