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

iii) HbA1c

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