Relationships Between Blood Thyrotrophin Level, Protein Bound Iodine and Free Thyroxine Concentration in Man Under Normal Physiological Conditions

A study of the thyroid-pituitary interrelationships in normal healthy subjects is carried out by measurements of plasma thyrotrophin (TSH), by a radioimmunoassay, protein bound iodine (PBI) and free thyroxine (T 4 ). It has been found that: – PBI and TSH significantly increase with age, which probably results from a peripheral hypometabolism, in spite of a normal free T 4 concentration; – there are circadian variations of free T 4 accompanied by a non significant inverse relationship of the plasma TSH level; – no important changes occur during the normal menstrual cycles but there is a significant increase in both PBI and TSH during pregnancy. The secretion of pituitary TSH seems to be finely regulated not only by the free plasma thyroxine concentration but also by the rate of metabolism of the thyroid hormones in the cell.