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

Summary

Increased serum concentrations of the aminoterminal propeptide of collagen III (PIIINP) are found in overt hyper-thyroidism. Thus, measurement of serum PIIINP might be useful as an early marker of tissue hyperthyroidism in patients with TSH suppressive thyroxine treatment. In a prospective study we eval-uated female patients followed in the thyroid outpatient clinic. Serum PIIINP concentrations were analysed in three groups: patients with TSH suppressive thyroxine treatment for more than 6 months (n = 33, TSH < 0.1 mU/1), patients with thyroxine sub-stitution for hypothyroidism for more than 6 months (n = 20, TSH 0.2-4.0 mU/1) and spontaneous hyperthyroid patients (n = 8, TSH < 0.03 mU/1, increased freeT4 and/or T3). Beside TSH, thyroid hormones 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 concentrations than the thyroxine substitution group (fT4 22 ± 4.8 pmol/l vs. 17 ± 2.6 pmol/l, T3 2.2 ± 0.4 nmol/l vs. 1.8 ± 0.3 nmol/l, p < 0.001) and also elevated serum SHBG values (77.6 ± 27.5 nmol/l vs. 58.4 ± 18 nmol/l, p < 0.01). However, serum PIIINP concentrations and the clinical score were very similar in both thyroxine treated groups (PIIINP in TSH suppression group 3.0 ± 0.67 µg/l vs. 2.8 ± 0.65 µg/l in the substi-tution 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 marker for detection of tissue hyperthyroidism in long-term thyroxine treated women with suppressed TSH.

Key words

Procollagen III peptide - TSH suppression - thyroxine treatment - hyperthyroidism