This is a companion piece to Thursday's post: Foods/Supplements-Vitamins: Iodine
"The daily Dietary Reference Intake recommended by the United States Institute of Medicine is ... 150 µg for adults ..." 
"The thyroid gland needs no more than 70 μg/day to synthesize the requisite daily amounts of T4 and T3. The higher recommended daily allowance levels of iodine seem necessary for optimal function of a number of body systems" 
"The International Council for the Control of Iodine Deficiency Disorders proposed that 150–299 μg/day is adequate to cover the thyroid requirement ..." 
"Seaweed is an important dietary component and a rich source of iodine in several chemical forms in Asian communities. Their high consumption of this element (25 times higher than in Western countries) has been associated with the low incidence of benign and cancerous breast and prostate disease in Japanese people."
"the average iodine consumption in the Japanese population is 1200–5280 μg/day versus 166 and 209 μg/day in the United Kingdom and the United States, respectively"
Symbols: [I−] = iodide, [I2] = iodine.
"the thyroid gland, the mammary gland, and the prostate can accumulate both types of iodine, and they are captured by different mechanisms. The thyroid gland, the lactating mammary gland, and the prostate exhibit a significant uptake of I−"
"... benefits have been found ... in human patients with early benign prostatic hyperplasia (stages I and II) where an 8-month Lugol (5 mg/day) supplement was accompanied by diminished symptoms and prostate-specific antigen values, and an increased urine flow rate ..." 
"Abraham and Brownstein argue that the human iodine requirement is 1,500 mcg per day (1.5 mg) which is difficult to achieve without using seaweed, iodized salt or supplementation." 
Sea sources:- ¼ oz dried seaweed: 4500 mcg; 3-4 oz cod: 100 mcg; 4 oz scallops: 135 mcg; 3 oz shrimp: 35 mcg)
Land sources depend on the iodine content of the soil, which may be minimal.
Supplements: Iodoral or Lugol's. (Iodoral is the pill form of Lugo's)
"The halogens or halogen elements ... are a group in the periodic table consisting of five chemically related elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At)."
Exposure to those other than iodine is problematic, since they compete in the body. e.g. fluorine in the municipal water supply can lower ones iodine status.
The literature is essentially concerned with the effect of halogen competition on the thyroid (hypothyroidism or cancer), but there would be some effect on other tissues that depend on iodine for health, if the thyroid was affected by halogen exposure.
"Fluorine is the world’s 13th most abundant element and constitutes 0.08% of the Earth crust." 
"The medicinal use of fluorides for the prevention of dental caries began in January 1945 when community water supplies in Grand Rapids, United States, were fluoridated to a level of 1 ppm as a dental caries prevention measure."
"The mechanisms through which fluoride exacerbates hypothyroidism include competitive binding with iodine, as well as synthesis obstruction of T3 and T4... Thus, fluoride-induced hypothyroidism is likely to be more common in iodine-deficient settings. Australian surveys indicate that the general Australian population is mildly deficient in iodine. Iodine-deficient children ingesting fluoridated water have been found to demonstrate intellectual deficits even at water fluoride levels of 0.9 ppm."
"In England, approximately 10% of the population live in areas with community fluoridation schemes and hypothyroidism prevalence can be assessed from general practice data." 
"We found that practices located in the West Midlands (a wholly fluoridated area) are nearly twice as likely to report high hypothyroidism prevalence in comparison to Greater Manchester (non-fluoridated area)."
" The three types of filters that can remove fluoride are reverse osmosis, deionizers (which use ion-exchange resins), and activated alumina. Each of these filters should be able to remove about 90% of the fluoride. By contrast, “activated carbon” filters (e.g., Brita & Pur) do not remove fluoride." 
There is a recent paper on "18F Sodium Fluoride PET/CT in Patients with Prostate Cancer" , which suggests to me that PCa cells are able to take up significant levels of iodine.
If chlorine is in one's water supply, the only way to avoid intake is to filter both drinking water & bathing water. During a shower, the skin supposedly absorbs whatever is in the water, at a rate of about one pint per gallon. In addition, we breathe in water vapor during a shower.
I understand, from when my kids kept fish, that water could be dechlorinated by letting it stand for 24 hours, but a Brita filter is claimed to remove 99% of the chlorine without the wait.
An unexpected problem with chlorinated water is that chlorine can react with polyphenols to produce antinutrients. The biggest problem, potentially, is for those drinking a lot of green tea, IMO:
"The Japanese scientists specifically mentioned that their studies showed that MX (unknown mutagens) are created by the reaction of chlorine with natural organic plant phytochemicals such as catechins, which are contained in tea, and flavonoids which are found in fruit."
In the U.S., the major concern is bromated flour:
"Potassium bromate (KBrO3), is a flour "improver" that strengthens dough and allows for greater oven spring and higher rising in the oven. Potassium bromate, commonly referred to as simply "bromate," is a slow-acting oxidizer, contributing its functionality throughout the mixing, fermentation and proofing stages, with important residual action during the early stages of baking. Azodicar-bonamide (ADA), potassium and calcium iodate, and calcium peroxide are rapid-acting oxidizers, while ascorbic acid (vitamin C) works at intermediate rates, but all release their activity in mixing and proofing. Bromate, when applied within the prescribed limits (15-30ppm), is completely used up during the bake leaving no trace in the finished product. However, if too much is used, or the bread is not baked long enough or at a high enough temperature, then a residual amount will remain.
"The primary concern regarding the use of bromate in baking is its demonstrated link to cancer in laboratory animals. It was first found to induce tumors in rats in 1982. However, instead of banning bromate outright, the FDA has since 1991 merely encouraged bakers to voluntarily stop using it, with somewhat limited success.
"Bromate has been banned in numerous countries, including the United Kingdom in 1990 and Canada in 1994. In addition, in 1991, California declared bromate a carcinogen under the state's Proposition 65. As such, baked goods sold in California have to bear a store-level cancer warning if they contain more than a certain level of bromate. As a result, most California bakers have switched to bromate-free processes."
 Anguiano B. Ledezma O. Juárez MA. Nuñez F. Aceves C. Therapeutic effect of iodine on human benign prostatic hyperplasia.. 14th International Thyroid Congress; Paris, France. Sep 11–16;; 2010. Abstract ITC2010-2585.