In a case of hypothyroidism with clubbing of fingers and toes, exophthalmos, myxoedema tuberosum and myxoedema pretibiale, serum T
4 value was low, but both values of serum T
3 and TSH were normal and serum LATS was potitive. Three months later both values of serum T
4 and T
3 decreased and serum TSH value increased without any treatment. These results show that serum T
3 as well as T
4 might be related to the feedback control of TSH secretion. In a case of Pendred's syndrome, serum T
4 value was low, values of serum T
3 and BMR were within normal and serum TSH value was high. Normal serum T
3 may have kept this patient in a state of compensatory hypothyroidism. In these patients, administration of 25-50 μg of
l-T
3 per day was sufficient to suppress the secretion of serum TSH from thyroid gland completely.
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