Investigating low thyroid stimulating hormone (TSH) level
BMJ 2013; 347 doi: https://doi.org/10.1136/bmj.f6842 (Published 20 November 2013) Cite this as: BMJ 2013;347:f6842
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Professor Weetman describes secondary hypothyroidism as rare. However, the incidence of TSH insufficiency following moderate to severe traumatic brain injury lies somewhere between 1% and 21% [1], and it can also occur after mere concussion [2]. There is therefore the possibility that up to 4,000 patients a year are afflicted in this way, and, given the current failure to diagnose these patients, the prevalence in the population may be far higher.
If one remembers that TBI is not the only possible source of secondary hypothyroidism and that other causes include pituitary adenomas, cranial radiation, genetic factors, autoimmune disease and even snakebite, it is clear that the index of suspicion should be higher than it is.
An additional spur for vigilance is the possibility that if the pituitary is dysfunctional, some of the patient’s symptoms may be due to deficiencies in other pituitary hormones.
[1] Tsagarakis S et al, Diabetes insipidus, secondary hypoadrenalism and hypothyroidism after traumatic brain injury: clinical implications, Pituitary, 2005;8(3-4):251-4. The annual incidence of TBI GCS<13 is around 21,000.
[2] Ives, J C et al, Hypopituitarism after multiple concussions: a retrospective case study in an adolescent male, J Athl Train. 2007 Jul-Sep; 42(3): 431–439.
joannalane@blueyonder.co.uk
Competing interests: No competing interests
The laboratory investigation of thyroid function is a major concern for the endocrinologist, laboratory consultant and the patient. When such tests are referred for geriatric population many do not yet have the normal values or reference range for many routine biochemical as well as for specialized tests. The ageing patient presents therefore a real challenge to the physician in requesting specific thyroid function tests to be carried out. It is therefore important for choosing and requesting relevant laboratory tests for thyroid status in particular and other biochemical investigations. The laboratory consultant has to be a bridge between the physician and the patient so that the benefit of proper testing will help the physician to diagnose in time and the patient to get proper treatment.
Competing interests: No competing interests
Re: Investigating low thyroid stimulating hormone (TSH) level
We greatly appreciate the informative review by Prof Weetman with regards a rational approach to the investigation of low thyroid stimulating hormone level (1). In our experience a commonly overlooked cause of a low TSH, particularly in hospital inpatients, is contrast induced thyrotoxicosis. Tens of thousands of patients each year receive iodine based contrasts through routine procedures such as CT scanning and coronary angiography. The iodine load given in these procedures can deregulate thyroid tissue and precipitate subclinical or even clinical hyperthyroidism (via the Jod Basedow phenomonen), particularly in elderly patients who may have previously undiagnosed multinodular goitres and a degree of thyroid autonomy. A study conducted in a geriatric hospital in 1993 by Martin and colleagues identified 25% of new onset hyperthyroidism in an elderly population over a 20 month period could be attributed to the recent administration of iodine contrast media (2).
This phenomenon has recently been highlighted in our hospital by the admission of an elderly patient with repeat episodes of cardiac chest pain. He had undergone coronary angiography two months previously. His biochemistry confirmed marked thyrotoxicosis on the background of a toxic multinodular goitre. This had precipitated new onset atrial fibrillation and a type 2 acute coronary syndrome (3).
On identification of a low TSH level careful questioning regarding previous radiological procedures should be undertaken. Thyroid dysfunction is most likely to occur 1-3 months post contrast and in most cases should resolve spontaneously without the need for treatment. However, as our unfortunate patient demonstrates this is not always the case, and specific treatments may be required if symptoms are present. Care should also be taken when administering iodine based contrasts to patients with pre-existing thyroid dysfunction.
1. Weetman AP. Investigating low thyroid stimulating hormone (TSH) level. BMJ 2013;347:f6842.
2. Martin FI, Tress BW, Colman PG, et al. Iodine-induced hyperthyroidism due to non-ionic contrast radiography in the elderly. Am J Med 1993;95(1):78-82.
3. Thygesen K, Alpert JS, White HD, et al. Universal definition of myocardial infarction. Circulation 2007;116(22):2634-2653.
Patient consent obtained.
Competing interests: No competing interests