Clinical data: a 34-year-old woman was referred for an evaluation of complaints suggesting hyperthyroidism. She delivered 8 weeks before the examination.

Palpation: both thyroids were moderately firm. There was no palpable nodule present.

Result of blood tests: moderate degree of hyperthyroidism with TSH-level 0.001 mIU/L, FT4 34.8 pM/L, FT3 9.60 pM/L.

Ultrasonography: the thyroid was practically intact. The vascularization was decreased.

Anti-TPO and TSH-receptor antibody tests resulted in normal value, 1 U/mL and 0 U/L, respectively.

We performed FNAC.

Cytological diagnosis: Hashimoto's thyroiditis.

We did not administer any medications to the patient.

Follow-up examinations: 3 months later hypothyroidism was detected (TSH 37.4 mIU/L, FT4 6.22 pM/L). In the next 1 year a gradual normalization of the hypothyroidism was observed and a complete normalization occured 17 months after delivery.

Comment: this situation is rare. The patient had post partum thyroiditis, but neither aTPO nor ultrasound was pathognomic. In order to establish the correct diagnosis, we had to perform FNAC.