Endocrine Abstracts

Objective: To report a patient with clinical hypothyroidism due to primary hypotriiodonaemia.

Design: Case report.

Main outcome: A 26-year-old woman presented with weight gain, cold intolerance, fatigue, mental lethargy, depression, long periods of amenorrhea and delayed ankle reflex relaxation. She was not on any medications. The (mean of two) thyroid stimulating hormone (TSH) concentrations was 2.5 (N: 3.8–5.5 mU/l), the free thyroxine (T4) 12.3 (N: 10.5–20 pmol/l) and the free triiodothyronine (T3) 3.15 (N: 3.50–6.50 pmol/l). Secondary hypothyroidism and recognized causes of the ‘low T3 syndrome’ were ruled out. The reverse triiodothyronine (rT3) was 180 (N: 120–540 pmol/l). Serum selenium and iodine levels were normal. The log/linear plot of the TSH versus T4 values revealed a sensitive TSH-negative feedback mechanism. Replacement with L-thyroxine resulted in clinical improvement, including restoration of normal menstrual cycles and ankle reflex relaxation, suppression of TSH to sub-physiologic levels at mid-normal T4 concentrations, a small increase in T3 and a marked rise in rT3. Computer modelling indicated the thyroid hormone concentrations to be most compatible with a decrease in thyronine deiodinase 2 (D2) activity.

Conclusion: A patient is described with hypothyroidism due to a decrease in circulating T3 levels not attributable to systemic illness, starvation, drugs, iodide excess or selenium deficiency. It is postulated to be due to a primary deficiency in peripheral T3 generation, most likely caused by an inborn error in D2 activity.