Endocrine Abstracts

A 31-year-old man originally from Ukraine attended the clinic having been started on Carbimazole 20 mg by his GP. His symptoms included intermittent palpitations, sweating and poor sleep and they had improved since starting anti-thyroid therapy. He was originally diagnosed with thyrotoxicosis in Germany in 1999 but was not treated and he re-presented in 2005. This time he was started on Carbimazole but stopped treatment on his own accord after a month because his symptoms improved. There was no family history of thyroid disease. He was a non-smoker and may have been exposed to radiation following the Chernobyl disaster. He was clinically euthyroid on examination. Thyroid function tests: TSH 8.29 mIU/L, FT4 22.8 pmol/L and FT3 6.7 pmol/L (see reference ranges in Table). Carbimazole was stopped and he underwent an MRI scan to investigate for TSHoma which was unremarkable. The rest of the pituitary screen also were in normal limits. A plan to investigate for TSH hormone resistance fell through due to appointment DNAs. After some time he re-attended having commenced carbimazole for elevated free thyroid hormones (fT4 57 pmol/L and TSH 0.01 mU/L). With carbimazole treatment blood tests improved with TSH 2.36 mIU/L, FT4 27 pmol/L and FT3 7.4 pmol/L. TSH-receptor antibody was mildly elevated at 2.5 IU/L consistent with auto-immune thyrotoxicosis. Genetic testing for TSH-resistance showed no causative mutation. Carbimazole was continued and titrated at follow-up appointments and subsequent biochemistry is shown below:

Biochemistry

The differential for elevated FT4 and normal TSH includes:

• Thyroxine replacement therapy

• Drug-induced

• TSH-oma

• Thyroid hormone resistance

• Non-thyroid illness

• Assay interference

We organised for the thyroid tests to be performed on an alternative platform (Abbott Architect, as opposed to ROCHE).

The values on the Abbott platform were very different, indicating that there is assay interference seen with the Roche assay.

We believe he has true thyroid disease, managed as Graves’ disease with carbimazole. Assay interference should be considered when thyroid results are confusing.