Endocrine Abstracts

Case history: A 40-year-old, primaparous woman was referred at 24 weeks’ gestation to Obstetric Medicine for review of abnormal thyroid function tests (TFTs).

Investigations: One year prior to pregnancy, TFTs included a thyroid stimulating hormone (TSH) level of 0.95 mU/l (local non-pregnant reference range 0.3–4.2 mU/l) and a free thyroxine level (fT4) of 7.3 mU/l (local non-pregnant reference range 9.0–23.0 mU/l). At 18 weeks’ gestation, routine TFTs showed a TSH of 1.00 mU/l (local reference range for 2^nd trimester 0.6–2.8 mU/l) and a fT4 of 7.3 mU/l (local reference range for 2^nd trimester 9.0–14.3 mU/l). She was prescribed 25 mcg thyroxine based on these results, and also took pregnancy multivitamins. She denied all symptoms of hypopituitarism and had a mid-day cortisol of 219 nmol/L. MRI pituitary showed a slightly enlarged gland, consistent with pregnancy. Thyroid assay interference was excluded by analysis at the supra-regional assay service.

Results and treatment: All results were reassuring, but with ongoing diagnostic uncertainty, 25–50 mcg thyroxine was continued. However her TSH remained normal, and fT4 remained < 8.0 mU/l. It was later concluded that she had isolated hypothyroxinaemia (IH). Healthy twin boys were delivered by caesarean section at 37+5 weeks’ gestation. At delivery her thyroxine was stopped; post-partum TFTs remained normal (TSH 0.52 mU/l, fT4 9.4 mU/L).

Conclusions: What is IH?: IH is defined by a fT4 below the lower limit of the trimester-specific reference range. It complicates 1–2% of pregnancies, with significant geographical variation (1).

What is the role of iodine supplementation?

IH is associated with iodine deficiency. Individual iodine status is difficult to assess, as urinary iodine has significant intra-individual variability (2); therefore cases of IH may benefit from empirical iodine supplementation.

What is the role of pituitary axis assessment?

Pituitary dysfunction can very rarely present as isolated hypothyroxinaemia. In pregnancy, thorough clinical assessment is essential, but routine intracranial imaging or biochemical assessment of the pituitary is not justified in all women with IH.

Is there a role for thyroxine treatment?

IH is not associated with adverse pregnancy outcomes so routine treatment with thyroxine is not recommended (3,4). If there is no clinical suspicion of pituitary dysfunction, it is reasonable to cease thyroxine treatment.

References: 1. Dong & Stagnano-Green, Thyroid 2018.

2. Wainwright & Cook, Annals of Clin Biochem. 2018.

3. Lazarus et al. Eur Thyroid J 2014.

4. Alexander et al. Thyroid 2017.