Evaluation of procollagen III peptide as a marker of tissue hyperthyroidism in long-term treated women with TSH suppressive doses of thyroxine

Increased serum concentrations of the aminoterminal propeptide of collagen III (PIIINP) are found in overt hyperthyroidism. Thus. measurement of serum PIIINP might be useful as an early marker of tissue hyperthyroidism in pat ients with TSH suppressive thyroxine treatment. In a prospective study we e valuated female patients followed in the thyroid outpatient clinic. Serum P IIINP concentrations were analysed in three groups: patients with TSH suppr essive thyroxine treatment for more than 6 months (n = 33, TSH < 0.1 mU/l), patients with thyroxine substitution for hypothyroidism for more than 6 mo nths (n = 20. TSH 0.2-4.0 mU/l) and spontaneous hyperthyroid patients (n = 8. TSH < 0.03 mU/l, increased freeT4 and/or T3). Beside TSH. thyroid hormon es and serum PIIINP we measured serum SHBG and a clinical score. Hyperthyroid patients had clearly elevated serum PIIINP and SHBG values and a higher clinical score when compared with other study groups (p < 0.001). Patients with TSH suppressive thyroxine treatment had higher fT4 and T3 co ncentrations than the thyroxine substitution group (fT4 22 +/- 4.8 pmol/l v s. 17 +/- 2.6 pmol/l, T3 2.2 +/- 0.4 nmol/l vs. 1.8 +/- 0.3 nmol/l. p < 0.0 01) and also elevated serum SHBG values (77.6 +/- 27.5 nmol/l vs 58.4 +/- 1 8 nmol/l, p < 0.01). However, serum PIIINP concentrations and the clinical score were very similar in both thyroxine treated groups (PIIINP in TSH sup pression group 3.0 +/- 0.67 mu g/l vs. 2.8 +/- 0.65 mu g/l in the substitut ion group. clinical score 2 +/- 1.8 pts. vs. 1.7 +/- 1.5 pts, p = n.s). In conclusion, serum PIIINP is not a reliable early market for detection of tissue hyperthyroidism in long-term thyroxine treated women with suppresse d TSH.