This section is from the book "Materia Medica And Therapeutics Inorganic Substances", by Charles D. F. Phillips. Also available from Amazon: Materia medica and therapeutics.
Iodine may be absorbed to some extent by the unbroken skin, if the local inflammation excited be not too severe. A dilute solution is therefore better absorbed than a strong, ir-ritant tincture. M. See maintained that the unbroken skin would not absorb iodine at all, and that any systemic effects following its application were due to absorption of the vapor by the lungs (Medical Times, i., 1874), but I believe the facts are as above stated. If a limb be painted with tincture of iodine, and covered with oil-silk, drops of colorless liquid may be found upon it after a few hours; this liquid contains the drug altered in some way by the perspiration (Gubler), and a similar alteration possibly occurs before its absorption. The drug may certainly be absorbed by the skin in a bath containing iodine and iodide of potassium; nor is there any question as to its free absorption from serous and mucous membranes. Iodide of potassium and other alkaline iodides are not absorbed as such, even when applied to the skin continuously in lotion or ointment; but, after being decomposed by the acids of the perspiration, or of lard, etc., they evolve free iodine, which may be absorbed, as proved by its appearance in the urine (Rabuteau). Iodide of ammonium is the alkaline salt most readily decomposed, but iodoform parts with its iodine still more readily (Lancet, i., 1863). Baehrach applied compresses with iodate of potash solution, 2 1/2 per cent., to the limbs, and in healthy subjects found iodine in the urine after fifteen minutes - in fever patients only after an hour or more (Centralblatt fur Medicin, ii., 1879). By mucous surfaces these compounds are easily absorbed; thus, when a suppository containing 20 gr. of iodide of potassium was placed in the vagina, 18 gr. were absorbed in twelve hours; glycerin diminished the rate of absorption, while a little free iodine increased it (British Medical Journal, i., 1878, p. 897). Serous membranes absorb iodides still more rapidly.
Taken into the stomach in small or moderate doses, iodine coagulates and combines with albuminous material, and is probably taken up in part as an albuminate, though a larger proportion combines with the soda of the gastric juice, and becomes iodide of sodium before being absorbed. Rabuteau thinks this combination with sodium occurs, to some extent, in the blood.
Alkaline iodides are either absorbed unchanged, or as iodide of sodium.
Hogyes has recently published observations on the absorption of iodoform. He states that, if introduced in an undissolved condition, the first step is its solution in whatever fatty matter may be at hand, e.g., the chyme in the intestine, and oily constituents of organic liquids in subcutaneous tissue and serous cavities. The oily solution of iodoform next gives up its iodine to any albuminous principle present. The iodide of albumen thus produced is speedily taken up into the blood, while a few minute coagula and oil-globules are left behind. The iodine is gradually eliminated from the system in combination with potassium or sodium (Ar-chiv far Exp. Pathologie und Pharmak., Bd. x., Hft. 3, 4).
Metallic iodides, such as those of iron, lead, or mercury, are decomposed, and also form iodide of sodium, which appears in the urine, while the metal passes by the bowel, or is deposited in the tissues. Absorption of iodides seems to be markedly promoted by ozonic ether (Day: Medical Times, i., 1871), by ammonia, and some other stimulants. Both iodine and the alkaline iodides are readily and rapidly eliminated by the different secretions, and may be detected in the saliva, the buccal and bronchial mucus, the tears, the milk, the perspiration, the urine, etc. R. W. Taylor has reported evidence of elimination of iodine by the skin in the case of a man with pityriasis, who took large doses of the potassium salt while wearing a starched shirt; he had profuse perspirations, and a dark coloration appeared on his back. After continued use of iodoform internally, iodine is clearly eliminated by the skin, as proved by the characteristic and unpleasant odor of the perspiration (Binz).
It would seem that almost all the iodine taken passes by the urine, for Scharlau recovered from that excretion 345 centigrammes out of 350 taken (Stille). According to Melsens, very little can be traced in the faeces; he suggests that the iodine that is excreted into the intestine is taken up again by the lining membrane before it reaches the rectum. Rabuteau found a small quantity in the faeces, so long as it was present in the other secretions; if diarrhoea occurred, the quantity was notably increased.
The rapidity of elimination varies with the quantity taken, a large dose giving evidence of its passage very quickly. Ranke found traces in the urine three and a half minutes after administration, and even sooner in the saliva. Nothnagel also found it early - in ten minutes in the latter secretion. Richardson found it in the urine within one minute of injecting tincture of iodine into an enlarged bursa, and, three minutes after breathing iodide of ethyl, iodine could be detected in the urine. It is an important practical point that elimination of this drug is complete sooner than that of many others. Dr. Balfour noted that even if large doses of iodide of potassium had been taken for many weeks, their elimination was complete within three or four days after ceasing to take them (Edinburgh Medical Journal, 1868). Dr. Duckworth, after a dose of 4 gr., found iodine in the saliva in five minutes, in the urine in twenty-five minutes; after twelve hours' interval it was still to be detected in both secretions, but after thirty-six hours in neither ("Bartholomew Hospital Reports," vol. iii.). Rabuteau, after 15 gr., found traces in the urine for three days; after 150 gr., for ten days; not afterward. The greater part was eliminated during the first day, little passed on the second, and scarcely a trace on the third; in the dog, elimination was somewhat slower. Claude Bernard, giving iodide of potassium, ceased to find it in the urine twenty-four hours afterward; that he detected it in the saliva for three weeks must be considered exceptional. Speck has stated that in Bright's disease the kidneys do not eliminate iodine, and Dr. Duckworth could not detect it in one case after giving 10 min. of the compound tincture; but 3 gr. of the iodide of potassium gave evidence of its presence, only much later than usual, namely, one, two, or three hours after administration.1 Baehrach, giving moderate doses of iodate of potash by the mouth to healthy subjects and to fever patients, traced the drug in the urine of both within fifteen minutes; but, on injecting it under the skin, elimination in the former occurred in five minutes, but in the latter forty minutes later (loc. cit.).
As will be noticed again under therapeutical action, iodides have a remarkable power of eliminating with themselves various metals and possibly organic poisons previously circulating in the blood or deposited in the tissues.