Biochemical transition during triphasic postpartum thyroiditis: confusion with central hypothyroidism

SUMMARY: Hypothyroidism occurring in the postpartum period can be due to pituitary or hypothalamic disease as in Sheehan’s syndrome and postpartum autoimmune hypophysitis or due to a primary thyroid disease as in postpartum thyroiditis. It is important that the correct diagnosis is ascertained because hypothalamic or pituitary disorders are often associated with other pituitary hormone deficiencies, especially life-threatening adrenal insufficiency or adrenal crisis. A combination of various symptoms and biochemical markers, especially serum thyroid-stimulating hormone levels dictate the initial diagnostic pathway. We present a case of a woman who presented with a 2-month history of tiredness and neck discomfort following delivery. A thyroid function test demonstrated results, which we initially interpreted as central hypothyroidism. Follow-up results indicated that this was in fact the transition period between the thyrotoxic phase and hypothyroid phases of postpartum thyroiditis. This case highlights the potential for diagnostic confusion between central hypothyroidism and postpartum thyroiditis. LEARNING POINTS: Postpartum thyroiditis affects one in twenty mothers within 12 months of delivery. The majority of patients have transient thyrotoxicosis only, some have transient hypothyroidism only, and the rest has a triphasic pattern (thyrotoxic, hypothyroid then a euthyroid phase). During the transition from the thyrotoxic phase to hypothyroid phase, when serum TSH is still suppressed, the biochemical results can resemble that of central hypothyroidism. If central hypothyroidism is suspected, then urgent diagnostic investigations should be carried out along with the assessment of adrenal function. There is a potential for diagnostic confusion between postpartum central hypothyroidism and postpartum thyroiditis; however, the obstetric history, clinical symptoms, and signs (headaches, breastfeeding, goitre, etc.) and serum adrenocorticotropic levels should help with the differential diagnosis.