Background: In laboratories employing 'front-line' sensitive
thyroid-stimulating hormone (TSH) measurement, it is generally accepted that a fully
suppressed serum TSH concentration (third-generation assay) alongside normal serum
concentrations of free thyroid hormones indicates subclinical hyperthyroidism.
However, other explanations are often provided for low but detectable serum TSH
concentrations, such as drug effects or non-thyroidal illness.
Methods: We investigated 25 consecutive ambulant individuals, identified
over an 18-month period as having low but not fully suppressed TSH concentrations
(third-generation assay; sensitivity 0.003 mIU/L) with additional free thyroxine
(T4), free tri-iodothyronine (T3) and thyroid microsomal
antibody estimations and thyroid isotope scanning (technetium).
Results: Concentrations of serum hormones (median, inter-quartile range)
were: TSH, 0.23, 0.17-0.26 mIU/L (reference range 0.34-5.6 mIU/L); free
T4, 14.6, 10.6- 17.6 pmol/L (reference range 10-25 pmol/L); free
T3, 6.1, 5.7-6.6 pmol/L (reference range 4.5-7.5 pmol/L). Thyroid
antibodies were negative in all but one individual. On isotope scanning, nine
individuals had hot nodules and ten individuals had multinodular goitres (MNG). Of
the six with normal scans, ultrasound scanning showed a definite MNG
(n = 1) and early MNG (n = 2).
Conclusions: A low but detectable serum TSH concentration, obtained
using a third-generation assay, found in an ambulant individual, is frequently a
pointer to underlying thyroid disease.
