Low but detectable serum thyroid-stimulating hormone concentrations in ambulant subjects not receiving thyroxine

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.

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 (T(4)), free tri-iodothyronine (T(3)) and thyroid microsomal antibody estimations and thyroid isotope scanning (technetium).

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 T(4), 14.6, 10.6- 17.6 pmol/L (reference range 10-25 pmol/L); free T(3), 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).

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.