If you've ever felt consistently exhausted, unable to lose weight despite trying, foggy-headed, cold when others aren't, losing hair, or just generally not right — and had your thyroid tested and been told it's "normal" — you're in a large and underserved group.
The thyroid gland sits at the base of your throat and produces two hormones — T4 (thyroxine) and T3 (triiodothyronine) — that regulate metabolic rate in virtually every cell of your body. When thyroid function is impaired, it affects everything: energy production, body temperature, cognitive function, digestion, mood, menstrual cycles, fertility, and cardiovascular health.
◈Key Facts
How thyroid hormone actually works
Your brain's pituitary gland sends thyroid stimulating hormone (TSH) to tell the thyroid to produce T4. T4 is largely inactive — it's a storage form. In the liver, kidneys, and other tissues, T4 is converted to the active form T3, which actually binds to cell receptors and does the work.
This means you can have technically normal TSH, normal T4, and still have poor thyroid function if the conversion to T3 is impaired. Standard thyroid panels often don't measure free T3 at all. This is one reason why many people with clear hypothyroid symptoms are told their labs are normal.
The chemical assault on your thyroid
Perchlorate
Perchlorate competes with iodine for uptake by the thyroid. Since iodine is what the thyroid uses to build thyroid hormone, anything that blocks iodine uptake directly reduces hormone production. Perchlorate is found in some water supplies and in certain leafy vegetables grown in affected areas (particularly rocket/arugula and spinach). It's also found in some bleached food packaging.
PFAS
PFAS (per- and polyfluoroalkyl substances) have been associated with thyroid disruption in multiple human epidemiological studies. They appear to interfere with both thyroid hormone transport and receptor binding. Given how ubiquitous PFAS are — cookware, food packaging, water-resistant clothing, stain-resistant carpets and furniture — this is a meaningful background exposure for most people.
Brominated flame retardants
PBDEs (polybrominated diphenyl ethers) are flame retardants that were used extensively in furniture foam, electronics, and textiles. They are chemically similar to thyroid hormone and compete for binding sites. While some PBDEs have been restricted or phased out in several countries, they persist in the environment and in older furniture and electronics. Legacy exposure continues.
Pesticides
Several pesticide classes, including organochlorines and some pyrethroids, interfere with thyroid function. Chlorpyrifos, once one of the most widely used agricultural pesticides, has been associated with altered thyroid hormone levels in human studies. Pesticide residue on food is the primary route of exposure for most people.
Soy in excess
Soy isoflavones, particularly in large amounts or in unfermented forms, can inhibit thyroid peroxidase — the enzyme that produces thyroid hormone. This isn't a reason to avoid all soy, but consuming large amounts of unfermented soy (protein powders, soy milk, edamame) alongside iodine insufficiency is a pattern worth being aware of for anyone with thyroid concerns.
The iodine connection
The thyroid needs iodine to make its hormones, and iodine deficiency is less rare in developed countries than is commonly assumed — particularly among people who avoid iodised salt and don't eat regular seaweed or seafood. Low iodine intake combined with high goitrogen or chemical exposure creates a compounding problem.
Selenium is also required for T4-to-T3 conversion. Brazil nuts (even one or two per day), seafood, and organ meats are the primary food sources.
Why women are disproportionately affected
Oestrogen upregulates thyroid binding globulin — the protein that carries thyroid hormone in the blood. Higher binding globulin means less free thyroid hormone available to enter cells. This is one reason women are more susceptible to thyroid dysfunction throughout their hormonal lives — during puberty, pregnancy, perimenopause.
The autoimmune component is also relevant. Hashimoto's is driven by an immune system that attacks thyroid tissue. Autoimmune conditions generally are more common in women (roughly 78% of autoimmune patients are female), likely due to the complex interaction between oestrogen and immune regulation.
A thyroid test that comes back "within range" is useful information but not the final word on thyroid function. The range is population-derived, and you might feel terrible at a TSH level that's technically normal for someone else.