The diagnosis of an underactive thyroid gland is confirmed with below normal serum thyroxine levels. Also known as myxoedema.

Baseline investigations - all patients

Full blood count

Useful indicator of general health and underlying disease.

Normocytic normochromic anaemia and eosinophilia may be seen with glucocorticoid deficiency.

Serum B12

B12 deficiency may indicate associated pernicious anaemia or coeliac disease.

Urea and electrolytes

Useful indicator of general health and underlying disease.

Hyperkalaemia and hyponatraemia may occur in glucocorticoid deficiency.

Liver function test

Liver function should be tested at baseline and may be abnormal with congestive cardiac failure.

Bone profile

Useful indicator of general health and underlying disease.

Hypercalcaemia may occur with thyrotoxicosis and glucocorticoid deficiency.

9am cortisol

Ensure patients have stopped sex steroid therapy, for example HRT or the combined oral contraceptive pill for six weeks prior to test.

A synacthen test is required to formally exclude Addison’s, however a 9am cortisol can be a useful screen. Levels below 250nmol/l demand thorough assessment, levels above 590nmol/l exclude glucocorticoid deficiency.

Thyroid stimulating hormone

Elevated thyroid stimulating hormone (TSH) implies hypothyroidism.

TSH levels within the normal range imply normal thyroid function, except in those with potential pituitary disease, in whom a normal or low TSH may be seen with hypothyroidism.

Ideal treatment of thyroid disease is reflected by control of symptoms and a serum TSH in the middle of the reference range - approximately 1mU/L.

Long term monitoring of known thyroid disease relies on serum TSH levels alone.

Free thyroxine

Thyroid stimulating hormone (TSH) and free thyroxine should be tested together in all patients suspected of having thyroid dysfunction.

Low thyroxine implies hypothyroidism and is associated with elevated TSH in primary hypothyroidism.

Low thyroxine occurring with low or normal TSH also requires further investigation as it may suggest pituitary pathology.

Ideal treatment of pituitary disease is reflected by control of symptoms and a serum thyroxine level in the upper half of the reference range.


ECG should be performed in all patients at baseline to document and confirm bradycardia or any other changes.

Thyroid auto-antibody screen

There are multiple different tests available for thyroid auto-antibodies, with varying levels of sensitivity.

Thyroid peroxidise (TPO) antibodies are commonly used as an initial screen and may be positive in patients with any auto-immune thyroid condition, with higher titres occurring in patients with Hashimoto’s thyroiditis and hypothyroidism.

Free liothyronine

Free liothyronine levels are not usually necessary for the assessment and treatment of thyroid dysfunction, as long term treatment should be guided by TSH levels.

Further investigations - selected cases only

Fasting lipid profile

Hypothyroidism may be associated with hyperlipidaemia, which may normalise with correction of thyroid biochemistry.

HbA1c and fasting glucose

Patients with auto-immune thyroid dysfunction are at increased risk of developing diabetes mellitus.

Thyroid ultrasound scan

Ultrasound scan is rarely performed in hypothyroid patients.

It is useful when the gland feels hard to assess whether there is a nodule within it or whether the whole gland is inflamed, for example when assessing for lymphoma within a Hashimoto’s gland.

Ultrasound scan is also essential in the assessment of a palpable thyroid nodule with any suspicious features and to guide how and whether to perform fine needle aspiration.

Ultrasound guided fine needle aspiration

Fine needle aspiration should be performed in all suspicious or solitary nodules.

Ultrasound guidance improves the safety of this procedure and increases the yield of this investigation by targeting specific areas of a nodule. Aspiration of cyst contents may also be performed at the same time as the sampling of solid components.

Ultrasound scanning may also identify patients in whom fine needle aspiration is not necessary.

Vitamin D levels

Patients with coeliac disease may well have low serum vitamin D levels.

In patients with known or suspected osteoporosis, serum vitamin D levels will guide whether routine calcium and vitamin D supplementation is enough, or whether specific high dose vitamin D replacement is required.

Thyroid radionucleide uptake scan

This is useful in patients with suspected thyroiditis.

Coeliac auto-antibody screen

Tissue transglutaminase and anti-gliadin antibodies should be checked if there is clinical suspicion of coeliac disease.

Short synacthen test

A synacthen test must be performed if there is clinical or biochemical suspicion of Addison's disease.