Congenital Goiter Sustaining Normal Level of Serum Triiodothyronine

Summary

We attempted to elucidate the deficient site of thyroid hormone biosynthesis in the thyroid gland and the mechanism of sustaining normal T₃ level in sera of a patient with congenital goiter. TY, a 8-yr-old boy, first noted the onset of a diffuse goiter at the age of 2. There was no clinical evidence of hypothyroidism except for the slight impairment of intellectual development and the awkward physical activity. BMR, T₃-RSU and T₄ showed low values (-13%, 20.8% and 2.2 μg/dl), but serum T₃ was normal (180 ng/dl). Serum TSH was 18 μU/ml. The intrathyroidal T₃ and T₄ were slightly low. Thyroidal ¹³¹I uptake was high, but KSCN discharge test was negative. Percent distribution of ¹³¹I labelled amino acids in the pancreatin digested thyroid homogenate was 17.4% in MIT, 33.4% in DIT and 11.3% in T₃ and T₄. Thyroid iodide peroxidase activities in mitochondrial and microsomal fractions were slightly low (19.6 and 26.8 (normal: 32 ± 3.0 and 37.4 ± 9.5) mμmoles/mg protein). The activity was not increased by the addition of hematin. Thyroglobulin was found to be normal. A biological half life of ¹³¹I labelled T₄ was shorter (3.5 days) than that of the normal. Electron microscopic examination exhibited the increment and expansion of endoplasmic reticulum in the follicular cell.

Low iodide peroxidase activity of this patient may correlate to low T₃ and T₄ level in the thyroid cell. Moreover, shortened biological half life of T₄ implies that normal T₃ level in serum is sustained by the accelerated conversion of T₄ to T₃ in peripheral tissues. The high TSH level might be induced by low T₄ level in serum. These may be a compensatory mechanism in an attempt to maintain euthyroidism in this patient.

Key-Words

Congenital Goiter - Thyroid - T ₄ Conversion - Iodide Peroxidase