Iodine uptake, inhibition

Perchlorates, which inhibit iodine uptake by the thyroid gland, reducing T4 and T3 synthesis, constitute another class of environmental chemicals that affect thyroid function. In the past, high doses of these compounds were used to treat hyperthyroidism. In recent years, there has been concern that contamination of drinking-water... 

D. inhibition of iodine uptake into the thyroid gland. 

Inhibition of Iodine Uptake into the Thyroid of Rats... 

PURPOSE AND RATIONALE Propyl-thiouracil (PTU) and a wide spectrum of drugs may inhibit thyroid hormone synthesis. Some of these drugs can be used for treatment of thyrotoxicosis. As consequence of thyroid peroxidase inhibition, the iodine uptake and content of the thyroid is decreased. This phenomenon is dose-dependent and may occur at lower doses than those increasing thyroid weight in rats (McGinty and Bywater 1945). The historical parameter of iodine content was replaced by measuring uptake and release of radio-iodine (131-1). 

The effects of amiodarone on thyroid function tests and in causing thyroid disease, both hyperthyroidism and hypothyroidism, have been reviewed in the context of the use of perchlorate, which acts by inhibiting iodine uptake by the thyroid gland (125), and there have been several other reviews (126-130). 

Nitropmsside is given by infusion, and its BP-lowering effect is directly related to the rate at which it is administered. When it is discontinued, the BP rises rapidly. Lethal cyanide poisoning may occur in patients with rhodanase deficiency, and thiocyanate may accumulate in patients with renal failure, thus inhibiting iodine uptake and causing hypothyroidism (see Figure 71). 

The use of K1 serves as an important remedy to protect from radioiodine exposure under nuclear accident conditions. In principle, under normal circumstances, excess iodine decreases sodium—iodide symporter (NIS) on the thyroid cell surface, thereby inhibiting further access for iodine into the thyroid. Excess iodide administration at the appropriate time decreases thyroid radioactive iodine uptake (RAIU) by increasing the amount of nonradioactive... 

Previously published reports have described both sub-clinical and overt thyroid dysfunction as a result of excess iodine ingestion. Moreover, iodine-induced hypothyroidism is not rare, at least in iodine-replete areas. The possible underlying mechanisms of this impairment consist of both the inhibition of thyroid function by excess iodine and the iodine-induced enhancement of thyroid autoimmunity. On the basis that an excess iodine intake may cause hypothyroidism, several study results support iodine restriction for the treatment of hypothyroidism. The success rate of dietary iodine restriction alone, without thyroid hormone replacement, is reported to be approximately 50-80% in patients with primary hypothyroidism due to Hashimotos thyroiditis and other causes. Relatively mild hypothyroidism, high radioactive iodine uptake and uptake, increased free plasma iodine and... 

Burke (1970) found that fluorine inhibited the sodium-potassium pump and resulted in dysfunction of the iodine pump indirectly, as well as low iodine uptake, thus decreasing serum thyroid hormone levels. KendaU-Taylor (1972) proved that fluorine decreased the production of 3 —5 -cyclic adenosine monophosphate by inhibiting adenylcyclase in the thyroid fluorine also blocked stimulation by TSH. It was also reported that fluorine eliminated 1 from the thyroid in the form of iodized protein. Few studies were reported on synergistic action of iodine and fluorine. 

Iodine supplementation at iodine deficient baseline inhibited iodine uptake and organization in the thyroid... 

The methods to enhance retention in thyroid tissue are limited. Although a high iodide concentration in the thyroid can inhibit the release of incorporated into thyroid hormone and iodinated Tg, it also causes a significant reduction in iodine uptake. However, lithium carbonate can increase retention in thyroid tissue without affecting uptake. 

I. Pharmacology. Iodine-131 is a product of fission reactions and is likely to be a major form of internal radioactive contamination following a major nuclear reactor accident or weapon detonation. Potassium iodide (Kl) serves to block thyroid gland uptake of the radioactive isotopes of iodine (to prevent thyroid cancer) by both diluting the radioactive iodine as well as filling the gland with nontoxic iodine, effecting inhibition of further uptake. For optimal protection, Kl should be administered before or at the time of exposure to inhaled radioactive iodines however, Kl may be initiated up to 3 or 4 hours after exposure. If the exposure to radioactive iodines persists, then even later administration may be of benefit. 

Shepherd s purse contains glucosinolates, compounds present in many plants in the Brassicaceae family (e.g., cauliflower and Brussels sprouts). These compounds have been shown to cross the placenta and to be present in breast milk (Panter and James 1990). At high concentrations, glucosinolates inhibit iodine uptake by the thyroid, although at lower concentrations cancer-preventive effects have been shown (Stoewsand 1995). 

At high doses, glucosinolate compounds inhibit iodine uptake although cancer-preventive properties have been shown at lower doses (Stoewsand 1995). 

Figure 7. The effect of various substituted benzimidazoles on inhibition of acid secretion and iodine uptake in relation to their lipophilicity.

---