Endocrine Abstracts

We describe below a 16-year-old male referred with neonatal hypothyroidism. He was commenced on levothyroxine at the age of 3 weeks. The TSH was >100 IU/l (normal: 0.27–4.7) and Total T4 was <10 nmol/l (normal: 60–150) and there was no uptake on thyroid isotope scanning. He was diagnosed as having congenital absence of thyroid tissue and given levothyroxine and the dose adjusted to maintain normal thyroid function. His mother was diagnosed with hypothyroidism 6 years prior to his birth.

At this presentation, he was biochemically euthyroid (TSH: 1.01, FT4: 16.7 pmol/l) and was on 100 mcg of levothyroxine daily. His thyroid peroxidase antibody level was 32 ku/l (normal: 0–34). As his mother was hypothyroid he was reinvestigated. His thyroglobulin assay was normal indicating the presence of thyroid tissue. After omitting levothyroxine for 7 days his thyroid function showed normal values (TSH: 1.1, FT4: 16.7 and FT3: 4.9 pmol/l). He was subsequently taken off levothyroxine and he remained euthyroid during next 8 months follow up (recent results: TSH: 1.52, FT4: 15.3, FT3: 6.1). His mother was tested for TSH receptor blocking antibody which was found to be high (405 U/l; normal <10). It was postulated that he had acquired hypothyroidism from the transplacental transfer of his mother’s TSH receptor blocking antibody. Such blocking antibodies produce a negative uptake isotope scan and so a negative scan does not confirm absence of thyroid tissue in the neonate.

This case is striking in that despite having levothyroxine for 16 years his thyroid gland was quick to produce normal levels of hormone after cessation of replacement treatment. Ideally TSH receptor blocking antibody should be looked for in the mother and the child in congenital hypothyroidism. If there is doubt about the diagnosis levothyroxine should be temporarily withdrawn at 3 years.