Investigating low thyroid stimulating hormone (TSH) levelBMJ 2013; 347 doi: https://doi.org/10.1136/bmj.f6842 (Published 20 November 2013) Cite this as: BMJ 2013;347:f6842
- Anthony P Weetman, professor of medicine
- 1Department of Human Metabolism, Faculty of Medicine, Dentistry and Health, University of Sheffield, Sheffield S10 2HQ, UK
The commonest causes of a low serum level of thyroid stimulating hormone (TSH) are excessive levothyroxine replacement, non-thyroidal illness, and subclinical hyperthyroidism.
In a patient who is not taking levothyroxine treatment, a low TSH level should prompt measurement of free thyroxine (FT4) and free triiodothyronine (FT3). If these are normal, the TSH level should be measured after six weeks to rule out non-thyroidal illness.
Subclinical hyperthyroidism is common in elderly people, and treatment may be indicated before progression to overt thyrotoxicosis to minimise bone loss and risk of atrial fibrillation
A 66 year old woman with chronic obstructive pulmonary disease visited her general practitioner with a history of persistent fatigue since a severe chest infection three weeks previously. The infection had responded to antibiotics during a four day hospital admission. Her general practitioner found no physical signs in the chest, although there was a small, multinodular goitre. A measurement of thyroid stimulating hormone (TSH) was requested, and the result was 0.06 mU/L (reference interval 0.4-4.0 mU/L).
What is the next investigation?
The presence of a goitre prompted examination for clinical signs of thyrotoxicosis, but sinus tachycardia, atrial fibrillation, fine tremor, eye signs (lid lag or retraction), and warm palms were absent. A drug history should also be taken: in this setting of a low TSH level, is the patient taking amiodarone or levothyroxine? Less common drug induced causes of a low TSH level are high dose prednisolone, recent treatment with carbimazole, and dopamine infusion.
Thyroid function tests
Laboratories vary in their testing strategy when a request for thyroid function tests is made.1 Because a serum TSH level within the reference interval excludes primary thyroid disease, and secondary (pituitary or hypothalamic) causes of thyroid dysfunction are uncommon, many laboratories measure only TSH if thyroid function tests are requested. Other laboratories will also measure free thyroxine (FT4) or will …