Endocrine Abstracts

Background/objective: The prevalence of low serum TSH in the general population is in the order of 2.5–3.9%, and increases with age. True subclinical hyperthyroidism (SH) (low/suppressed serum TSH, normal serum fT4 and fT3) is less common. In one population based study of 1193 subjects, 66 (5.5%) were found to fit the criteria for SH. Of these, 75% had serum TSH 0.05–0.5 mIU/l, of whom 76% normalised within 12 months. At 12 months 87.5% subjects with serum TSH <0.05 mIU/l showed no change in thyroid function and 1 progressed to overt hyperthyroidism. Management of patients with subclinical hyperthyroidism is uncertain. Our aim was to assess the characteristics of subjects with SH in a secondary care thyroid clinic.

Method: Casenote review of 164 patients with low serum TSH referred to our thyroid clinic. We compared data at time of referral and at 12 months.

Results: Twenty-four patients (3 males) with SH were identified. Of these, 14/24 (58%) had serum TSH <0.05 miu/l. The indications for measuring thyroid hormones were varied, routine health screen being the most common indication, 7 patients, followed by weight loss, 5 patients.

By 12 months, 4 patients (16.6%) had progressed to overt thyrotoxicosis. 1 had reverted to normal biochemistry. Of those who progressed, 2 had serum TSH concentrations >0.05 miu/l at referral. No patient developed new atrial fibrillation and none suffered any bone fracture.

Conclusion: Within the clinic progression to overt hyperthyroidism is more common and reversion to normality appeared to be less likely than has been reported for the general population. Outcome at 12 months could not be predicted by clinical, biochemical or scan characteristics at presentation. No adverse outcome was seen. Until a consensus on the management of SH is reached, it appears to be appropriate to monitor such patients.