Low as opposed to fully suppressed serum TSH levels in ambulant subjects; frequently a pointer to thyroid autonomy
S Chatterjee1, BP O'Malley1, DE Price2, AM Fielding2 & R Aitken1
In laboratories employing 'front-line' sensitive TSH measurement, it is generally accepted that fully suppressed TSH levels (3rd generation assays) alongside normal free thyroid hormone levels indicate subclinical hyperthyroidism. However, low but not suppressed levels are often labelled as non-thyroidal illness.
We investigated 25 successive ambulant individuals, identified over an 18 month period as having low but not fully suppressed TSH levels (3rd generation assay; sensitivity 0.01-0.03 mIU/L) with additional free T4, free T3 and thyroid microsomal antibody levels and thyroid isotope scanning (technetium).
Serum TSH levels (mean [SD]) were 0.22[0.26] mIU/L (NR 0.34-5.6 mIU/L), free T4 levels 14.7[4.2] pmol/L (10-25pmol/L), and free T3 levels 6.1[0.8]pmol/L (4.5-7.5pmol/L). Thyroid antibodies were negative in all but one individual. On isotope scanning, 9 individuals had hot nodules and 10 individuals had multinodular goitres (MNG). Of the six with normal scans, ultrasound scanning showed a definite MNG (N=1) and early MNG (N=2).
Thus a low as opposed to fully suppressed TSH is frequently a pointer to underlying thyroid disease.