Endocrine Abstracts

Background: Subclinical hypothyroidism (SCH) in women planning pregnancy is is associated with adverse maternal and fetal outcomes, particularly when thyroid autoantibodies are present. Several professional societies, recommend earlier intervention in such cases. However, a lack of consistent international guidelines makes clinical decision-making difficult in practice.

Case Presentation: A 28-year-old woman with type 1 diabetes mellitus, well controlled on a hybrid closed-loop insulin pump, underwent routine thyroid function testing at her annual review. Initial results in January 2025 were normal (TSH 0.30 mU/l, FT4 18.8 pmol/l). Six months later, repeat testing revealed TSH 8.9 mU/l and FT4 11.6 pmol/l. The patient had discontinued her contraceptive patch six months earlier while planning pregnancy, though menstruation had not resumed. She denied hypothyroid symptoms such as lethargy, weight gain, or cold intolerance. Follow-up thyroid function three weeks later demonstrated partial improvement (TSH 5.6 mU/l, FT4 12.8 pmol/l). Levothyroxine therapy was initially deferred given the downward trend. Six weeks later, TSH was 4.0 mU/l with stable FT4 at 12.8 pmol/l. Her case generated debate within the consultant group. Some clinicians advocated starting levothyroxine immediately due to her pre-conception status, while others advised awaiting thyroid antibody testing. Antibody results revealed markedly positive thyroid peroxidase antibodies (217 IU/mL) and negative TSH receptor antibodies (1.5 IU/l). Considering these findings, along with her intention to conceive, levothyroxine 50 μg daily was initiated.

Discussion: This case highlights the diagnostic and therapeutic uncertainty of SCH in young women planning pregnancy. The presence of thyroid autoantibodies increases the likelihood of progression to overt hypothyroidism, providing a strong rationale for treatment. Guideline recommendations vary. NICE suggests treating SCH (TSH 4–10 mU/l on two occasions) only if the patient is symptomatic. Whereas, BTA recommends treatment when thyroid antibodies are positive or TSH exceeds 10 mU/l, while emphasizing that even mild TSH elevations in pregnancy or pre-conception should be managed as thyroid failure due to associated risks for both mother and fetus. ATA recommends treatment if TSH >10, except in patients with increased Cardiovascular risk. This case underscores the need for individualized decision-making, particularly in antibody-positive but asymptomatic women. It also raises the question of whether treatment would still be appropriate in antibody-negative women with TSH levels above 2.5 mU/l during pre-conception.

Conclusion: Managing subclinical hypothyroidism in women planning pregnancy is rarely straightforward. This case shows how fluctuating thyroid results, positive antibody status, and differing guideline recommendations can make decisions challenging.