July 27, 2017
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Abnormal thyroid function tests

Thyroid dysfunction may cause relatively non-specific symptoms such as fatigue, malaise, mood swings, bowel disturbance and palpitations. Whilst the classically thyrotoxic or hypothyroid patient can be diagnosed clinically, thyroid function tests are commonly performed in patients experiencing such symptoms without convincing signs of thyroid disease, and similarly the biochemical picture may be uncertain.

The predictive value of abnormalities in thyroid function are derived from a community based study in North England - the "Whickham Survey". This demonstrated that the annual incidence of hypothyroidism was 3.5/1000 women. The odds ratio for developing hypothyroidism were:

  • 8 for women and 44 for men with an isolated increase in TSH
  • 8 for women and 25 for men with positive anti-thyroid antibodies
  • 38 for women and 173 for men with an increase in TSH and positive antibodies

Positive thyroid microsomal antibodies were present in 21% of women aged 55-65 years and these women have an annual risk of developing hypothyroidism of 2.6%.

Over 20 years 1/3 of women with an isolated increase in TSH will become clinically hypothyroid.

Hyperthyroidism is much less common with an annual incidence of 0.8/1000 women.

Isolated suppressed TSH may occasionally indicate early hyperthyroidism. However treatment is probably only of benefit in such patients if there is associated atrial fibrillation.

Interpretation of thyroid function tests in the elderly can be problematic. TSH may be suppressed in normal elderly subjects and thyroid biochemistry is particularly susceptible to intercurrent illness in elderly people, although a "sick euthyroid" pattern can be seen in any patient with significant acute illness. The commonest sequence of abnormalities is a decrease in thyroid hormone concentrations with normal TSH; the fall is more marked with total than "free" hormones and for T3 than T4. There is subsequently a delayed rise in TSH as thyroid hormone levels return to normal. However any change in TSH levels may be seen in association with non-thyroidal illness. The best advice is not to test thyroid function in the acutely unwell patient unless there is a strong clinical suspicion of thyroid dysfunction, since giving thyroid hormone to patients with non-thyroidal illness has not been shown to be of benefit.

Drugs affecting TSH concentration

decrease TSH increase TSH

bromocryptine

clomiphene

carbamazepine

iodides

corticosteroids

lithium

cyproheptadine

metoclopramide

dopamine

morphine

heparin

phenothiazines

levodopa

 

thyroxine, tri-iodothyronine

 

References:

  • Vanderpump MP, Tunbridge WM, French JM, Appleton D, Bates D, Clark F, Grimley Evans J, Hasan DM, Rodgers H, Tunbridge F, et al. The incidence of thyroid disorders in the community: a twenty-year follow-up of the Whickham Survey. Clin Endocrinol (Oxf) 1995;43:55-68.
  • Rae P. Farrar J. Beckett G. Toft A. Assessment of thyroid status in elderly people BMJ 1993;307:177-80.
  • Sawin CT, Geller A, Wolf PA, Belanger AJ, Baker E, Bacharach P, Wilson PWF, Benjamin EJ, D'Agostino RB. Low serum thyrotropin concentrations as a risk factor or atrial fibrillation in older persons. NEJM 1994;331:1249-1252.
  

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