Endocrine Tests and Investigations
i) Capillary blood glucose
This refers to a one-off blood test which measures the circulating glucose level in your blood.
– It is important to know whether the patient is fasting or non-fasting when interpreting the results.
– If fasting –> Normal range = 3.9-7.1mM
Impaired Fasting Glucose (IFG) = fasting glucose sample >6.1mM but <7.0mM
ii) Oral Glucose Tolerance Test (OGTT)
This is a test which measures the body’s ability to metabolise a glucose load. It specifically tests the ability of the pancreas to release insulin and how responsive the tissues are to this hormone.
– A fasting blood glucose is taken followed by a 75g glucose load.
– The blood glucose is then repeated 2 hours later.
Impaired Glucose Tolerance (IGT) = OGTT > 7.8mM but <11.1mM
This is a measure of the glycosylated haemoglobin in the bloodstream.
– HbA1c is proportional to glucose concentration in the blood.
– It is used to measure the average blood glucose over the past 2-3 months.
– It is used to measure glycaemic control in those with DM every 3-6 months until stable then 6 monthly.
The level of HbA1c is dependent on blood glucose but also affected by RBC lifespan, therefore:
– Conditions which decrease RBC life span give a lower than expected HbA1c e.g. haemolytic anaemias.
– Conditions which increase the RBC life span give a higher than expected HbA1c e.g. megaloblastic anaemia, iron deficiency anaemia, or splenectomy.
– May also get a raised HbA1c from people taking medication that causes hyperglycaemia e.g. steroids.
iv) Thyroid Function Tests
These principally measure free T4 and T3 as well as TSH.
– If hyperthyroidism suspected –> Ask for suspected T3, T4 and TSH. All have decreased TSH due to negative feedback with raised T4
– If hypothyroidism suspected –> Ask for only T4 and TSH. TSH is raised with low T4
Other Thyroid tests:
– Thyroid antibodies – antithyroid peroxidase or antithyroglobulin antibodies are seen in autoimmune disease
– TSH receptor antibody – this is detected in Graves’ disease
– Serum thyroglobulin – this is useful in monitoring treatment of thyroid carcinoma
– Ultrasound scan – this test is used to distinguish cystic from solid nodules
– Isotope scan – this is used to detect the cause of hyperthyroidism and to detect thyroid nodules/metastases