Endocrine Abstracts

A 58-year-old man was diagnosed with subclinical hypothyroidism on routine biochemistry in primary care. His initial thyroid stimulating hormone (TSH) was 10.7 mu/l, Free T₄ (fT₄) 24 pmol/l and thyroid peroxidase antibody negative. He was treated with levothyroxine and doses escalated due to elevated TSH (9.6–20.58 mu/l) and low fT₄ (12.0–24.9 pmol/l). Despite good adherence to treatment his TSH was persistently raised and hence he was referred to our hospital.

At referral in August 2007 he was taking 200 mcg of levothyroxine. He felt unwell on these doses. Repeat thyroid function tests (TFTs) at our hospital were consistent with biochemical thyrotoxicosis with TSH 0.44 mu/l and T₄ 62.3 pmol/l. His levothyroxine doses were gradually reduced to 25 mcg/day in April 2008.

Mr P lived within the catchment area of a different hospital where blood tests conducted by his GP were sent for analysis.

When he was seen in the consultant clinic in October 2008, a careful review of his biochemistry revealed the following discrepancies.

Heterophile antibodies were suspected and his thyroxine was discontinued.

Blood samples from the GP catchment hospital were sent to a tertiary centre and showed positive heterophilic antibodies to TSH. A sample from our lab, which was sent to Birmingham was negative for antibodies to FT₃, FT₄ and TSH showing that the Roche assay was not interfered with.

This case highlights that even the commonest endocrine problem can be riddled with complexity. Studies reveal that up to 3.4% of patients have significant levels of heterophilic antibodies leading to spurious results and unwarranted therapy. This raises the question as to whether one should obtain repeat TFTs using a different assay in all cases of subclinical hypothyroidism?