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Low Serum Triiodothyronine (T₃) and Hypothyroidism in Anorexia Nervosa

Section of Endocrinology, Department of Medicine, School of Medicine Auckland, New Zealand
University of Southern California, School of Medicine Los Angeles, California 90033

Reprint requests to: H. K. Ibbertson, Section Endocrinology, Department of Medicine, School Medicine, Auckland, New Zealand.

Measurements of serum thyroid hormones were compared in 22 patients with typical anorexia nervosa and 22 euthyroid control subjects. Serum total triiodothyronine (T₃) was (mean ± SE) 62.1 ± 7.1 ng/100 ml in anorexia patients and 115.2 ± 8.4 ng/100 ml in control subjects (P < 0.001). Serum adjusted thyroxine (T4Adj) was significantly different in the anorexia (7.1 ± 0.4) and control (8.2 ± 0.4) groups. Serum T₃ was subnormal in 63% and T4Adj subnormal in 36% of the 22 anorexia patients. The mean serum T₄/T₃ in anorexia patients (158 ± 19) was higher than that in the control subjects (88 ± 5.5, P < 0.005) or in 18 patients with hypothalamic or pituitary hypothyroidism (77.9 ± 10.1, P < 0.001). Following weight gain in 6 anorexia patients, there was a significant rise in serum T₃ without change in T₄Adj concentration.

The Achilles reflex half-relaxation time (ART) in 38 anorexia patients was 348.6 ± 10 msec compared with 280 ± 30 msec in 168 normal agematched subjects (P < 0.001), and was prolonged (>340 msec) in 65% of these 38 patients. In 18 anorexia patients with measured ART, T₃ and T4Adj, the mean ART was longer (376.1 ± 20 msec) in 10 with subnormal T₃ than in 8 patients with a normal T₃ (294.7 ± 13.2 msec, P < 0.01). There was no significant difference in the mean ART between patients with a normal or low serum T₄Adj. Administration of oral T₃ 40 /βg/day for 4 weeks to 11 anorexia patients caused a significant reduction (P < 0.001) in mean ART of 108.7 ± 9.6 msec compared with 17.7 ± 3.3 msec in 18 normal subjects.

There was a normal peak serum TSH and a rise mean total serum T₃ of 47 ± 12 βg/100 ml (range 11–100 ng/ml) in 7 of 8 patients following 200 /xg of iv thyrotropin releasing hormone (TRH). The fall in serum TSH was delayed in 6 patients. Assessment of hypothalamic control of thyroid function in 3 patients using the method of thyroidal iodide release (TIR) showed impairment of the normal diurnal variation and response to administered glucocorticoids.

In the absence of a space-occupying pituitary lesion, the TRH and TIR data suggest a central inhibition of thyroid function, possibly by impairment of hypothalamic TRH release. In addition, probable decrease of peripheral T₄ to T₃ conversion leads to low serum T₃ concentrations. The prolonged basal ART and the marked ART reduction in response to T₃ administration is attributed correction of tissue thyroid hormone deficiency the anorexia patients.

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