Studies on the Absorption of Orally Administered Thyroid Hormones

Abstract

According to the Hays' method, the rates of intestinal absorption of thyroxine (T₄) and triiodothyronine (T₃) were estimated from the serum ¹²⁵I/¹³¹I ratio after simultaneous administration of oral ¹²⁵I-T₄ or ¹²⁵I-T₃ and intravenous ¹³¹I-T₄ or ¹³¹I-T₃.
The subjects employed were 11 normal euthyroid volunteers, 16 untreated and 9 treated hyperthyroid patients, 10 untreated and 9 treated hypothyroid patients, 3 patients with liver disease, 5 patients with congestive heart failure, one patient with anorexia nervosa, one patient with postgastrectomic afferent loop syndrome and one patient with gastric cancer.
In the present study, about 15, μCi of ¹³¹I-T₄ or ¹³¹I-T₃ was given intravenously and, at the same time, an equal dose of ¹²⁵I-T₄ or ¹²⁵I-T₃, diluted with 0.9% saline containing 80 ml of 1% bovine serum albumin was given orally.
Blood samples were taken 2, 4, 24, 48 and 72 hours after the dose and duplicate serum samples were counted for ¹²⁵I and ¹³¹I in a dual channel well-type scintillation counter.
Percentage of the dose of each isotope per liter of serum was calculated and percent absorption was calculated as

'The treated patients' indicates the patients who were in euthyroid status by treatment, in this study.
The average values of maximal rate of absorptioh of T₄ were 67.59±19.20% /48 hr for euthyroid subjects, 73.32±18.67% / 4 hr for untreated hyperthyroid patients, 74.63±13.88% / 48 hr for treated hyperthyroid patients, 69.52±18.60% / 24 hr for untreated hypothyroid patients and 66.69±14.67% / 48 hr for treated hypothyroid patients.
The average values of maximal rate of absorption of T₃ were 85.36±14.95% / 4 hr for euthyroid subjects, 92.51±11.53% / 4 hr for untreated hyperthyroid patients, 95.87±10.07% / 4 hr for treated hyperthyroid patients, 84.91±17.23% / 4 hr for untraeted hypothyroid patients and 79.71±12.34% / 4 hr for treated hypothyroid patients.
The rates of absorption of T₄ or T₃ in liver disease, congestive heart failure and gastric cancer tended to be lower than that in euthyroid subjects.
The average maximal rate of absorption of T₄, 67.6%, in euthyroid subjects was quite similar to the Hays' average value, 68.0%, but that of T₃, 85.4%, in euthyroid subjects was lower than the Hays' value, 95.4%.
The average maximal rates of absorption of T₄ and T₃ in untreated hyper- and hypothyroid patients were almost the same as that in euthyroid subjects, but the time required to attain maximal absorption was shorter in hyperthyroid and longer in hypothyroid patients than in euthyroid subjects.
The average maximal rate of absorption of T₄ or T₃ in treated hyper- and hypothyroid patients were almost the same as that in untreated ones. The time required to attain the maximal rate of absorption of these hormones tended to be normalized by treatment both in hyper- and hypothyroid patients, although it was longer in treated hyperthyroid patients than in untreated ones, and, shorter in treated hypothyroid patients than in untreated ones.
The rate of maximal absorption of the hormones observed in the patients with liver disease, congestive heart failure and gastric cancer tended to be lower than that in euthyroid subjects and it was thought that such low rates of absorption of the hormones may be partly due to a stasis of the portal system and stagnation and edema in the intestine.
It was observed that the absorption of T₃ was quicker and its rate of maximal absorption was higher than that of T₄ in every group investigated.