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Postgrad Med J 2001;77:29 doi:10.1136/pmj.77.903.29
  • Review

Subclinical thyrotoxicosis

  1. A C Al-Abadi
  1. Department of Diabetes and Endocrinology, Clinical Sciences Centre, Northern General Hospital, Herries Road, Sheffield S5 7AU, UK
  1. Dr Al-AbadiAmira{at}ammori.freeserve.co.uk
  • Received 8 November 1999
  • Accepted 27 April 2000

Subclinical thyrotoxicosis may be defined as a low serum thyrotrophin (TSH) concentration in an asymptomatic patient with normal serum free thyroxine (T4) and triiodothyronine (T3) concentrations. The secretion of TSH may be suppressed even in the presence of normal serum thyroid hormone levels. This reflects the highly sensitive response that the pituitary gland mounts to minor changes in serum free T4 and T3concentrations inside the normal range of the population—exemplified by the log-linear relation between serum TSH and thyroid hormone concentrations.1

In this review I shall address the various disorders characterised by a low serum TSH and how to differentiate them from subclinical thyrotoxicosis. I also discuss the causes of subclinical thyrotoxicosis, its potential pathophysiological and clinical consequences, and its management.

The introduction in the mid-1980s of sensitive assays for TSH allowed the measurement of serum TSH concentrations well below the normal range of 0.5 to 4.0 mIU/l. Unlike the first generation radioimmunoassays, which have a sensitivity of about 0.1 mIU/l, the new immunometric second and third generation assays have greater sensitivities of approximately 0.05 and 0.005 mIU/l, respectively, with about a 10-fold increase in sensitivity per generation.1 2 The thyrotrophin releasing hormone (TRH) stimulation test has since played a diminishing diagnostic role.3

Causes of low serum TSH concentrations

The main causes of low serum TSH concentrations are overt thyrotoxicosis, non-thyroidal illness, secondary (central) hypothyroidism, physiological causes, and subclinical thyrotoxicosis.

OVERT THYROTOXICOSIS

Overt thyrotoxicosis is characterised by high serum free T4 and T3 and very low serum TSH concentrations, which are undetectable in more than 95% of patients, even with third generation assays,4 and it is associated in the majority of patients with overt symptoms and signs of thyrotoxicosis. Two small subgroups of thyrotoxic patients are characterised by a normal serum T4 in association with either …

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