Evidence for a slow tissue adaptation to circulating thyroxine in patients with chronic L-thyroxine treatment.

We measured serum procollagen-III-peptide in 67 women with long-term L-thyroxine treatment and compared the results with age-matched controls. The strong correlation between serum free thyroxine and procollagen-III-peptide concentrations previously found after 6 months of L-thyroxine treatment was not found after long-term treatment. There were slightly higher procollagen-III-peptide concentration values in those chronically treated patients who had high free thyroxine levels but this increase was less marked than in patients previously studied after short-term treatment. An increase in procollagen-III-peptide concentration reflects an increased biosynthesis of collagen III, which is present in connective tissues throughout the body, and our findings may be explained by slow tissue adaptation to increased levels of thyroxine. We conclude that the increased thyroxine levels found in L-thyroxine-treated patients are of less clinical importance than thought previously. We also conclude that peripheral markers of thyroid hormone peripheral effects such as procollagen-III-peptide may be of less use than thought previously due to this slow tissue adaptation to changes in thyroxine concentration.