Endocrine Abstracts

Introduction: Congenital hypothyroidism is caused by abnormal development or function of the thyroid gland. Early detection through heel prick screening and treatment prevents irreversible adverse neuro developmental outcome. The national screening program for congenital hypothyroidism in the UK has extremely low false positive rates. We describe 4 siblings with falsely raised TSH related to maternally transmitted macroTSH.

Case summary: 4 siblings currently aged 10, 8, 3 years and 19 months presented after birth with elevated TSH on day 3–5 heel prick. These were confirmed with further blood biochemistry. All were started on Levothyroxine. Their mother, currently 39 years old has a long history of elevated TSH with normal T4 levels and clinically euthyroid, not on treatment. There is no family history of thyroid disorder. Her THRβ gene sequencing did not show any mutations. The older 3 siblings were noted to have normal thyroid ultrasound and thyroid uptake on scan and successfully discontinued thyroxine by age 30 months and remain euthyroid. When the 4th child was born simultaneous samples were sent from the mother and child for further testing. Gel filtration chromatography confirmed the presence of high molecular massTSH immunoreactivity in the samples of the mother and PEG recovery was abnormal at 21 (27–20) % in this baby. Once confirmed as the cause of falsely elevated TSH the levothyroxine was discontinued in the 4th sibling at age 1 year. Follow up bloods and development remains normal at 19 months.

Discussion: Increased immunoreactive TSH, in this case, is likely to be due to Macro-TSH, an immunoglobulin TSH complex. This can accumulate in circulation, simulating a laboratory picture of subclinical hypothyroidism. Multiple TSH assays can be affected. Macro-TSH can be detected by immuno-subtraction and dilution studies and confirmed by gel filtration chromatography. While the heel prick test is extremely sensitive and specific for congenital hypothyroidism this rare cause should be considered specially in case where consecutive siblings are tested positive.