Thyroid stimulating hormone (TSH) is measured as a first line test for all thyroid function requests as it is the most sensitive marker of thyroid function. If the TSH is abnormal (either below or above the reference interval), then a free thyroxine (fT4) test is automatically added on to aid interpretation. If you require fT4 despite a normal TSH level, please contact the laboratory on rduh.bsaddon@nhs.net. Additional tests for thyroid dysfunction may be added on by the Duty Biochemist upon review.
In certain clinical circumstances it is possible to request TSH, fT4 and/or fT3 directly in GPOCS ICE:
Elevated TSH with a low FT4:
In a patient with signs and/or symptoms of hypothyroidism:
“Suspect a diagnosis of primary hypothyroidism if TSH levels are above the normal reference range (usually above 10 mU/L) and FT4 is below the normal reference range.”
“Use clinical judgement to interpret thyroid function test (TFT) results, especially if TFTs do not match the clinical presentation.
From <https://cks.nice.org.uk/topics/hypothyroidism/diagnosis/assessment/>
Elevated TSH with FT4 within the normal range:
“Suspect a diagnosis of subclinical hypothyroidism if TSH levels are above the normal reference range and FT4 is within the normal reference range.
In non-pregnant people, repeat TFTs 3–6 months after the initial result to exclude other causes of a transiently raised TSH and to confirm the diagnosis.”
From <https://cks.nice.org.uk/topics/hypothyroidism/diagnosis/assessment/>
Suppressed TSH (less than 0.1 mIU/L) with FT4 and FT3 within the reference range:
“Suspect a diagnosis of subclinical hyperthyroidism if the TSH level is below the normal reference range and FT3 and FT4 levels are within the normal reference range.
Repeat TFTs 3 months after the initial result to exclude other causes of a transiently suppressed TSH and to confirm the diagnosis.
If on repeat testing there is a persistently low TSH level with normal FT4 and FT3 levels, the diagnosis is confirmed.”
From <https://cks.nice.org.uk/topics/hyperthyroidism/diagnosis/assessment/>
“Consider seeking specialist advice on managing subclinical hyperthyroidism in adults if they have:
2 TSH readings lower than 0.1 mIU/litre at least 3 months apart and evidence of thyroid disease (for example, a goitre or positive thyroid antibodies) or symptoms of thyrotoxicosis.
Consider seeking specialist advice on managing subclinical hyperthyroidism in all children and young people.”
Suppressed TSH (less than 0.1 mIU/L) with FT4 and/or FT3 above the reference range:
In a patient with sign(s) and/or symptom(s) of hyperthyroidism:
“Suspect a diagnosis of overt hyperthyroidism if the TSH level is low and FT4 and/or FT3 levels are raised above the normal reference ranges.”
From <https://cks.nice.org.uk/topics/hyperthyroidism/diagnosis/assessment/>
“Be aware that thyroid function tests (TFTs) may produce misleading results in certain clinical situations, and clinical judgement should be used when interpreting results”
From <https://cks.nice.org.uk/topics/hyperthyroidism/diagnosis/assessment/>
Levothyroxine replacement for hypothyroidism:
“Aim to maintain TSH levels within the reference range when treating primary hypothyroidism with levothyroxine. If symptoms persist, consider adjusting the dose of levothyroxine further to achieve optimal wellbeing, but avoid using doses that cause TSH suppression or thyrotoxicosis.”
“Be aware that the TSH level can take up to 6 months to return to the reference range for people who had a very high TSH level before starting treatment with levothyroxine or a prolonged period of untreated hypothyroidism. Take this into account when adjusting the dose of levothyroxine.”
Specimen Labelling Procedure
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