Iodine, thyroid and the menstrual cycle: why Latin America does not have one single answer

The thyroid does not make hormones out of nothing. To produce T4 and T3, it needs iodine.

That sounds like a simple fact, almost from a school textbook. But in female hormonal health it has a much more interesting consequence: iodine can change the amount of thyroid signal available to the ovary, corpus luteum, endometrium and metabolism. And when that signal changes, the menstrual cycle can change too.

The mistake is turning this into a flat sentence: "you need more iodine".

Recent evidence from Latin America says something more complex. The region does not have one single iodine problem. It has at least two.

On one hand, there are women and subgroups with insufficient exposure. On the other, there are countries and subregions where population exposure appears high or excessive. In the middle are millions of women with a more useful question than "does my country have iodine?":

Does my real pattern of food, salt, hormonal stage and thyroid susceptibility put me in deficiency, excess or instability?

Iodine is thyroid substrate, not a generic nutrient

The thyroid takes up iodide from the blood, uses it with the TPO enzyme to iodinate thyroglobulin and form T4 and T3. T4 works as a circulating reserve. T3 is the more biologically active form in tissues.

That system does not live separately from the menstrual cycle.

Thyroid signaling participates in the follicle’s response to FSH, estradiol production, corpus luteum function, progesterone synthesis and endometrial receptivity. It also intersects with insulin, SHBG, cortisol and available energy.

That is why a woman can have reproductive symptoms before having clinically evident hypothyroidism.

It does not mean every irregular cycle is iodine. It means iodine input can be one piece of the thyroid-reproductive system, especially when it overlaps with other factors: chronic stress, irregular sleep, insulin resistance, positive anti-TPO, pregnancy, lactation or perimenopause.

What changed in Latin America

For decades, the dominant story was iodine deficiency, goiter and iodized salt.

Iodized salt worked. Pretell and Pearce, in a historical review published in 2024, report that 92% of households in Latin America consume adequately iodized salt. It is one of the region’s great silent public health achievements.

But that achievement does not mean the problem has disappeared at the individual level.

The Iodine Global Network Scorecard 2025 shows a heterogeneous region. Colombia appears with a median urinary iodine level in school-age children of 407 micrograms/L, classified as excessive. Costa Rica appears in the excessive range with 314 micrograms/L. At the same time, Haiti reports 77 micrograms/L in women aged 15 to 49 years, an insufficient range, and Nicaragua appears insufficient in non-pregnant/non-lactating women.

Brazil illustrates the point more precisely. In the EMDI-Brazil 2025 study, 1,891 pregnant women had an adequate median urinary iodine level: 186.7 micrograms/L. If you only look at the median, Brazil seems fine.

But the distribution tells another story: 36.7% had deficiency, 28.7% were above the requirement and 3.6% were in excess.

The conclusion is not "Brazil is fine" or "Brazil is doing badly".

The conclusion is that the national average can hide opposite extremes.

Deficiency and excess can feel similar

When there is too little iodine, the thyroid has less substrate to produce T4 and T3. The body may compensate by raising TSH, sometimes within the range still reported as "normal". If that compensation is not enough, the margin of thyroid signaling in sensitive tissues drops.

In the ovary and endometrium, that could translate into slower folliculogenesis, a less robust corpus luteum, lower luteal progesterone or more variable cycles. In a woman trying to conceive, that margin can matter.

Mills and colleagues published a prospective study in Human Reproduction in women trying to become pregnant. Women with iodine/creatinine below 50 micrograms/g had 46% lower fecundability per cycle compared with women with sufficient iodine. This does not prove that taking iodine resolves fertility. It does show that iodine status can touch reproduction before the story is clinically obvious.

Excess works differently.

When iodine load is high, the thyroid activates protective mechanisms such as the Wolff-Chaikoff effect: it temporarily slows iodine organification to avoid producing too much hormone. Most people escape that blockade. But in susceptible women, especially with Hashimoto’s, positive anti-TPO or nodularity, high exposure can make the system more unstable.

There is also an immunological route. Greater iodine organification can increase thyroid oxidative stress and thyroglobulin/TPO antigenicity in predisposed people. That is why excess is not neutral when there is autoimmunity.

The practical result: low iodine and high iodine can share nonspecific symptoms. Fatigue. Energy changes. Feeling cold or hot. Anxiety or palpitations in some cases. Cycles that become less predictable.

The difference is not resolved by intuition. It is resolved with context, measurement and clinical judgment.

The new phenotype: wellness without iodine

There is an urban pattern that classic surveillance may not capture.

A woman stops using regular iodized salt because it seems "processed" to her. She switches to pink salt, artisanal sea salt or kosher salt. She reduces salty ultra-processed foods. She lowers dairy. She eats little egg, little fish and almost no shellfish. Sometimes she also reduces salt heavily because she fears sodium.

From the outside, her diet may look "healthier".

But if those sources were her main iodine input, her exposure may fall. And if she is pregnant, lactating, in perimenopause or has high thyroid demand, the margin narrows.

This is not an argument against eating better. It is an argument against assuming that "natural" always means "better covered".

Sea salt is not automatically a reliable source of iodine. Pink salt is not necessarily iodized. Regular iodized salt can be, for many women, an important functional source.

The other extreme: high exposure and autoimmunity

In countries or subregions with high ioduria, the question changes.

If a woman consumes a lot of iodized salt, broths, consommés, salty processed products and lives in an area with high population exposure, the main risk may not be deficiency. It may be excess or volatility.

That matters especially if there is a family history of Hashimoto’s, positive anti-TPO, fluctuating thyroid symptoms or a history of nodules.

Public communication often treats iodine as if it were a linear variable: low is bad, high is good.

In thyroid physiology, the curve is more like a U. Too little can cause harm. Too much can too.

What Lua can observe without diagnosing

Lua Care does not replace labs, ultrasound or medical consultation. Nor should it tell a woman to take iodine.

What it can do is something the clinical system rarely captures: observe longitudinal patterns.

Not just "you have fatigue".

But:

• what type of salt you normally use;
• whether you eat eggs, dairy, fish, bread, broths or salty ultra-processed foods;
• in which phase of the cycle fatigue appears;
• whether the pattern coincides with spotting, a short luteal phase, constipation, cold intolerance, hair loss or dry skin;
• whether there is a family history of Hashimoto’s or positive anti-TPO;
• whether there is pregnancy, lactation, perimenopause or recent dietary changes.

With that, an exposure map can be built. Not as a diagnosis, but as a better-prepared conversation.

At Lua Labs we provisionally call it IEVS: Iodine Exposure Volatility Score. The idea is not to say "you are deficient" or "you are in excess". The idea is to identify whether a woman’s pattern suggests low exposure, high exposure or instability, and cross it with symptoms and cycle.

That kind of signal can be useful for bringing a more concrete question to the doctor:

"My TSH looks normal, but I switched to non-iodized salt, I eat little dairy/egg/fish and my luteal phase got shorter. Does it make sense to review iodine, thyroglobulin, free T4, free T3, anti-TPO or thyroid history?"

Or the other way around:

"I live in an area with high iodine exposure, I consume many salty products and I have a family history of Hashimoto’s. Does it make sense to review thyroid autoimmunity before assuming my symptoms are just stress?"

The answer is not supplementation. It is stratification.

With iodine, acting without knowing can be counterproductive.

If the problem is deficiency, increasing exposure could be relevant, but the clinical decision depends on context, pregnancy, lactation, labs, diet and individual risk.

If the problem is excess or autoimmunity, adding iodine can worsen the system.

That is why the right strategy for an app like Lua is not to recommend iodine. It is to map exposure, symptoms and time.

The question is not:

"Is iodine good for hormones?"

The question is:

"In this woman, with this diet, this salt, this hormonal stage, this cycle and this thyroid history, does iodine seem to be acting as insufficient substrate, as immunological excess or as a stable variable?"

That is the difference between a tips app and a longitudinal hormonal intelligence app.

Lua Care is a longitudinal hormonal intelligence app made in Mexico for women in LATAM. It learns with you, observes real patterns across cycles and helps you bring better questions to your consultation.

Medical disclaimer: This article is educational and does not constitute a diagnosis or clinical recommendation. If you have thyroid symptoms, positive anti-TPO, pregnancy, lactation, infertility, irregular cycles or questions about iodine, consult a health professional.

Scientific references

• Pretell EA, Pearce EN. (2024). The Journal of Nutrition, 154(12), 3856-3867. doi:10.1016/j.tjnut.2024.10.009
• Iodine Global Network. (2025). Global scorecard of iodine nutrition in 2025 in the general population based on school-age children.
• Candido AC et al. (2025). Nutrients, 17(17), 2753. doi:10.3390/nu17172753
• Silva DLF et al. (2024). Biological Trace Element Research, 202(7), 3025-3036. doi:10.1007/s12011-023-03909-4
• Beer RJ, Herran OF, Villamor E. (2021). The Journal of Nutrition, 151(4), 940-948. doi:10.1093/jn/nxaa392
• Mills JL et al. (2018). Human Reproduction, 33(3), 426-433. doi:10.1093/humrep/dex379