THYROXINE 
Absorption from the Gastrointestinal Tract 
The bulk of the earlier work on absorption of thyroxine and thyroxine -containing 
products has been largely concerned with the relative rates of absorption of thyroxine 
in its several forms. Little quantitative data on actual absorption were obtained, On the 
basis of comparisons of urinary iodine excretion with total thyroidal iodine administered, 
an estimated 16-28 percent absorption was suggested. Absorption from isolated intestinal 
loops of 90 percent and from isolated stomach of 40 percent have been observed. Recently 
Monroe and Turner (4) reinvestigated this problem by comparing the amount of orally 
administered thyroxine needed to maintain normal thyroid size in thiouracil-treated 
chicks with the amount required by parenteral administration. These tests indicated an 
absorption of 15 to 20 percent. These same workers using goats and comparing input 
with fecal output calculated absorption of 78-88 percent. When re-excretion of absorbed 
thyroxine into the gut is considered, this high value would suggest that thyroxine is 
almost completely absorbed from the gut in this species. Mixner and Lennon (3) have 
also estimated the efficiency of absorption of thyroxine from thyroprotein to be 8.3 to 
17.2 percent. 
Accumulation in Tissues 
After parenteral administration, thyroxine appears to be distributed about equally 
between blood, liver, and the remaining body tissues. No quantitative data exist on actual 
concentration of thyroxine in tissues as affected by intake. The half-life of thyroxine in 
humans is estimated at 7 to 12 days. Thyroxine rapidly disappears from blood and the 
major points of accumulation are then in the liver, kidney, and intestinal tract. Pre- 
sumably muscle and other edible tissues are in equilibrium with the blood and would 
reflect changes in thyroxine content of blood produced by administration of thyroxine. 
The feeding of thyroprotein containing thyroxine increased blood protein-bound -iodine 
values from a normal level of 2.0 to 5.0 Ug./100 ml. to levels exceeding 25.0 ug./100 ml. 
(2). Administration of thyroxine containing products (iodinated casein) to cows apparently 
does not cause detectable accumulation in the milk. There is evidence, however, that 
thyroxine may be transmitted from mother to child during the suckling period (7). As 
much as 10 percent of the activity of labeled thyroxine was observed in the ovocytes of 
laying hens (9). 
Metabolism of Thyroxine 
The major known metabolic change in thyroid hormone is one of deiodination. Most 
of the liberated iodine is excreted in the urine. Deiodination appears to occur primarily 
in the liver and kidney. The disposal and the nature of the noniodinated residues are 
practically unknown. Presumably they could be metabolized to tyrosine or to propionic 
acid. Two propionic acid derivatives, tetraiodopropionic acid and tri-iodopropionic acid 
are present in small quantities in bile and urine but not in feces. Phenolic glucuronides 
of thyroxine and tri-iodothyroxine are also present in bile but do not appear in blood, 
urine, or feces. This latter mechanism apparently represents a detoxification reaction 
and also involves an entero-hepatic circulation of the thyroidally active components, 
Excretion of Thyroxine 
In experiments with laboratory animals from 50 to 80 percent of an injected dose of 
thyroxine appears in the gastrointestinal tract within 2 hours. The major pathway of 
entrance is by way of the liver and bile. Part of the thyroxine at least is reabsorbed. 
About two thirds is ultimately excretedinthe feces as thyroxine mostly and the remainder 
metabolized and excreted as iodide in the urine. 
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