Lesson 2: Signs and Symptoms of Hyperthyroidism
Subclinical Hyperthyroidism
Disorders of hyperthyroidism range from mild subclinical conditions to severe conditions of thyroid storm. While subclinical hyperthyroidism is related to very slight increases in thyroid hormone levels, in thyroid storm, thyroid hormone levels are high but not unusually high. Thyroid storm occurs when a secondary event, such as infection, alters the way in which the body’s cells respond to thyroid hormone.
Subclinical hyperthyroidism is defined as a low or suppressed TSH in the presence of normal thyroid hormone levels. Usually, when TSH is low, doctors order a measurement of free T4 with the FT4 test. However, if the FT4 result is normal, it’s important to run a FT3 test. In some instances, the FT3 level is elevated and the FT4 normal. This condition is called T3 thyrotoxicosis.
In subclinical hyperthyroidism, the pituitary gland halts TSH production when it sees that thyroid hormone levels are rising. In doing so, the pituitary regulates thyroid hormone levels, keeping them from rising too high. However, even a slight increase in thyroid hormone can cause mild symptoms of hyperthyroidism.
Other conditions besides hyperthyroidism can cause TSH to fall. To tell if subclinical hyperthyroidism is the cause most doctors will order other tests. The RAI uptake test is often used but levels in subclinical hyperthyroidism may be normal and not reflect a slight increase in activity. Tests for thyroid antibodies, particularly a TSI level, are more helpful in determining if subclinical Graves’ disease is present.
Subclinical hyperthyroidism is as likely to resolve spontaneously as it is to progress. Many patients are able to reverse subclinical hyperthyroidism by reducing dietary iodine. Aggressive therapies aren’t generally recommended. If symptoms are present, beta blocker medications, such as propranolol or atenolol, may be used.
Most patients are asked to return to their doctor in 6 weeks to 6 months for repeat testing, or sooner if symptoms develop or worsen. Elderly patients with subclinical hyperthyroidism are more likely to have atrial fibrillation or other symptoms. In this case, a very low dose of an anti-thyroid drug, either 50-100 mg PTU or 5-10 mg methimazole, is used to reduce thyroid hormone levels without allowing them to fall too low.
For a better understanding of subclinical hyperthyroidism and the disease course in autoimmune thyroid disease, students are asked to read pages 72-75 in Chapter 3 of Graves' Disease, A Practical Guide by Elaine Moore and pages 28-31 in Autoimmune Diseases and Their Environmental Triggers by Elaine Moore
Discussion Topics
Can subclinical hyperthyroidism be diagnosed in patients on anti-thyroid drugs?
Does a low TSH always mean subclinical hyperthyroidism?
