triiodothyronine--reverse and Hypopituitarism

To investigate the respective role of triiodothyronine (T3) and thyroxine (T4) in the regulation of TSH secretion, we studied the action of sodium ipodate and propylthiouracil (PTU) in 11 athyreotic patients. The LT4 replacement dose was adjusted to obtain, in each patient, a normal basal TSH level and a normal TSH response to TRH. In the 5 ipodate-treated patients (single 6 g oral dose), the mean serum T3 level fell by 64% below the baseline value and serum rT3 rose 180% above the baseline. The free T4 index (FT4I) did not change whereas the mean serum TSH concentration increased 280% above baseline values. In the 6 PTU-treated patients (250 mg orally every 6 h for 10 days), serum T3 levels fell 33%, serum rT3 increased up to 82% and the FT4I did not change. The mean serum TSH concentration increased 68% above the baseline value. Thus, the mean percentage increase in serum TSH was less in PTU- than in ipodate-treated patients (68% vs 280%). Statistical analysis of the correlation between the serum T3 decrease (delta T3) and the serum TSH (delta TSH) increase demonstrated that for the same T3 diminution, the ipodate-treated group displayed higher increase of TSH than the PTU-treated patients. In the rat, PTU interferes with the 5'-deiodination of T4 in the liver and kidney but not in the pituitary, while ipodate appears to have the same effect in all tissues. If this holds true for human subjects, our data strongly suggest that circulating T4 (through its intrapituitary conversion to T3) shares with serum T3 the capacity to regulate TSH secretion in man.

Topics: Adult; Female; Humans; Hypopituitarism; Ipodate; Male; Middle Aged; Pituitary Gland; Propylthiouracil; Thyrotropin; Thyroxine; Triiodothyronine; Triiodothyronine, Reverse

The plasma concentrations of thyroxine (T4), free thyroxine (free-T4), triiodothyronine (T3), reverse triiodothyronine (rT3), TSH and thyroxine-binding globulin (TBG) were measured in 19 children suffering from idiopathic growth hormone deficiency. Blood was taken before and one month after growth hormone treatment. Ten patients were hypothyroid (group 1) and 9 were euthyroid (group 2). The basal T3 and rT3 levels correlated well with the T4 concentrations. Free-T4 levels were very low in all the hypothyroid patients and proved to be the most reliable index of TSH deficiency. TBG concentration was high in th hypopituitary patients regardless of their thyroid function. Following growth hormone treatment T4, free-T4 and rT3 levels fell in both groups. The T3 concentration rose in group 1 but no change was seen in group 2. There was a significant correlation between the changes of T4 and T3, such that the increase in T3 level was greatest in those with only a slight reduction of T4 concentration and no T3 increase was seen with more marked T4 decreases. The plasma TBG concentration is enhanced in growth hormone deficiency causing relatively high T4 values. Growth hormone treatment reduces T4 secretion and affects the peripheral metabolism of thyroid hormones resulting in an increase of T3 and a reduction of rT3 concentration.

Topics: Adolescent; Child; Child, Preschool; Female; Growth Hormone; Humans; Hypopituitarism; Male; Thyroid Hormones; Thyrotropin; Thyroxine; Thyroxine-Binding Proteins; Triiodothyronine; Triiodothyronine, Reverse