Hyperthyroidism (or thyrotoxicosis) refers to an overactive thyroid gland. Subclinical hyperthyroidism (also known as subclinical overactive thyroid gland or mild or severe subclinical hyperthyroidism) is diagnosed when thyroid function tests show normal thyroid hormone levels of FT3 and FT4 but low or suppressed levels of thyroid-stimulating hormone (TSH).1 Occasionally the FT3 levels may be high.1
Mild subclinical hyperthyroidism is more common than severe subclinical hyperthyroidism.1
Subclinical hyperthyroidism can have an exogenous (internal) cause or endogenous (external) cause. Exogenous causes are more common i.e. taking too much levothyroxine or caused by the drug amiodarone. Endogenous causes may include Graves’ disease, toxic multinodular goitre, toxic adenoma and certain types of thyroiditis.1,2
It is important to note that low TSH levels are very common in the first trimester of pregnancy.1
Most people who suffer from subclinical hyperthyroidism don’t present with any of the symptoms of hyperthyroidism (an overactive thyroid). However, in some cases, symptoms are similar to that of hyperthyroidism but usually milder and could include:2
Subclinical hyperthyroidism can occur for various reasons. When testing takes place, it is therefore imperative to eliminate certain causes including hyperthyroidism, pituitary or hypothalamic disease, euthyroid sick syndrome and drug-induced suppression of TSH.1
TSH levels would be tested to ascertain if they are low or suppressed because undetected TSH is more likely to progress to overt hyperthyroidism whereas TSH levels below the range but not completely suppressed may be transient. Detailed medication history should be taken to assist in the diagnosis.1 If your FT4 and FT3 are not tested it might be an idea to ask for these as your FT3 level may be high which is an indication of the early stages of hyperthyroidism.1
Once exogenous causes have been ruled out, and further TSH tests confirm that the levels are not transient, treatment can be considered on a case by case basis.3
Should medication be prescribed, treatment of subclinical hyperthyroidism is usually very similar to that of overt hyperthyroidism. Often beta-blockers are prescribed or antithyroid drug therapy such as carbimazole or metabolite methimazole, usually in low doses.1
The aim of medication would be to restore the euthyroid state and avoid any side effects. In some cases (i.e. nodules, ablative treatment), surgery or radioiodine treatment are considered.1
It is common for mild cases of subclinical hyperthyroidism to resolve themselves and go into remission over a period of time (usually just a few months). Regular testing will be required to monitor TSH levels.1 People with undetectable TSH levels have a higher risk of progressing to overt hyperthyroidism 4 which will require treatment.
If you believe you may have subclinical hyperthyroidism book an appointment with your GP to discuss this. Read the guidance listed in the references below, make notes and take it with you to your appointment.
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Date updated: 01.09.21 (V1.6)
Review date: 27.08.22
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