Here is what the WHO has to say about populations in the world who have iodine deficiency and how they determine that, plus how it affects TSH. It should be noted that most doctors and medical professionals assume that people do not have iodine deficiency (without checking) simply because of the existence of iodized salt. It might be true, if you can confirm that all packaged processed and restaurant foods use iodized salt exclusively. But I don't think that's the case, and I've never been asked by a doctor, "Do you use iodized salt?" I think it's a mistaken assumption.
I became interested because in my case, my TSH didn't go down until my T3 was "too high" and my T4 was considered "low" so what they say about overlap is really on the ball. They don't mention genetics but I'd bet that those of us with more Neanderthal DNA might have different needs of some nutrients.
Thyroid stimulating hormone (TSH)
The pituitary secretes TSH in response to circulating levels of T4. Serum
TSH rises when serum T4 concentrations are low, and falls when they
are high. Iodine deficiency lowers circulating T4 and raises the serum
TSH, so iodine-deficient populations generally have higher serum TSH
concentrations than do iodine-sufficient groups.
However, the difference is not great and much overlap occurs between
individual TSH values. Therefore, the blood TSH concentration
in school-age children and adults is not a practical marker for iodine
deficiency, and its routine use in school-based surveys is not recommended.
In contrast, TSH in neonates is a valuable indicator for iodine deficiency.
The neonatal thyroid has a low iodine content compared to that of
the adult, and hence iodine turnover is much higher. This high turnover,
which is exaggerated in iodine deficiency, requires increased stimulation
by TSH. Hence, TSH levels are increased in iodine-deficient populations
for the first few weeks of life – this phenomenon is called transient
The prevalence of neonates with elevated TSH levels is therefore a
valuable indicator of the severity of iodine deficiency in a given population.
It has the additional advantage of highlighting the fact that iodine
deficiency directly affects the developing brain.