Woman checking her temperature in the morning as part of a thyroid health check

9 Signs Your Thyroid Could Be Underactive

Slow, steady weight gain. Feeling the cold more than everyone else in the room. Low energy that just won't lift. It's easy to put these things down to getting older, being busy, or simply having an "off" few months. But they can also point to something more specific, and genuinely worth knowing about: an underactive thyroid.

Four times as many women as men are diagnosed with an underactive thyroid, which makes this a particularly relevant topic to understand, especially since it can quietly be at the root of gradual weight gain that otherwise seems to have no obvious cause.

What your thyroid actually does

Your thyroid gland sits in your neck and helps control your metabolism. It produces two hormones, thyroxine and triiodothyronine, which are released when a signal travels from your hypothalamus and pituitary gland, with the pituitary also producing thyroid-stimulating hormone (TSH).

Think of your thyroid a bit like a thermostat: it regulates your body temperature by releasing these two hormones, which in turn control how quickly your body burns calories and uses energy. An underactive thyroid, or hypothyroidism, happens when there isn't enough thyroid hormone circulating in your body, caused either by the pituitary gland not producing enough TSH, or by the thyroid itself not working quite as it should.

The 9 signs to check

Here's a simple checklist worth going through. If you find yourself answering "yes" to four or more of the following, it's worth discussing your thyroid with your doctor:

  • Has your weight gone up gradually over months for no apparent reason?
  • Do you often feel cold?
  • Are you constipated?
  • Are you depressed, forgetful or confused?
  • Are you losing hair, or is it drier than it used to be?
  • Are you having menstrual problems?
  • Are you having difficulty getting pregnant?
  • Have you noticed a lack of energy?
  • Are you getting headaches?

Answering yes to several of these is a genuine reason to visit your doctor for a blood test. It's a starting point for a proper conversation, not a substitute for one.

Why a normal blood test doesn't always tell the full story

An underactive thyroid problem can sometimes go undiagnosed even when it's genuinely present, because blood tests can occasionally look normal. This happens when your thyroid gland is producing enough hormone, but the cells that are supposed to respond to it aren't quite picking it up the way they should.

Because of this, a simple at-home temperature test can be a useful additional check. It involves taking your temperature first thing in the morning, before getting up, drinking anything or visiting the bathroom, using a thermometer placed under your armpit. Do this for three consecutive days, and if you have periods, on days two, three and four of your cycle, since temperature naturally rises after ovulation. If your average temperature over those three days falls below 36.4°C (97.6°F), this may point to a sluggish metabolism linked to an underactive thyroid.

This test isn't a replacement for a proper diagnosis, but it can be a useful conversation starter with your doctor if your blood test result doesn't seem to match how you're actually feeling.

The nutrients involved in thyroid function

Iodine

Essential for your thyroid gland to function healthily, with good dietary sources including seafood, saltwater fish and seaweed. One important caution: taking too much iodine can actually make an underactive thyroid condition worse, so this is best approached with guidance from a qualified practitioner rather than adjusted on your own.

Selenium and tyrosine

Selenium is a genuinely useful mineral for thyroid hormone function, with low selenium levels linked to underactive thyroid problems. Tyrosine is an amino acid that, together with iodine, your body uses to actually make thyroxine and triiodothyronine.

B vitamins, zinc and manganese

B2, B3 and B6 are necessary for normal thyroid hormone production, meaning a shortfall can affect thyroid function and, in turn, your metabolism. Zinc works alongside vitamins A and E in the manufacture of thyroid hormone, while manganese is involved in enzyme systems relevant to thyroid hormone function as well as fat metabolism.

Goitrogens: foods to cook well

Certain foods, called goitrogens, can block the uptake of iodine and potentially worsen an underactive thyroid problem when eaten raw and in excess. These include turnips, cabbage, peanuts, soya, pine nuts and millet. Cooking these foods well, and eating them in moderation, genuinely reduces this effect.

Bringing the nutrients together

Sourcing iodine, selenium, tyrosine, B vitamins, zinc and manganese separately, at appropriate levels, is difficult to manage, and iodine in particular needs careful handling. Advanced Thyroid Support combines these nutrients in balanced amounts formulated specifically for thyroid function. As always with thyroid concerns, get a proper diagnosis from your doctor first, and discuss supplementation with a qualified practitioner, particularly if you're already on thyroid medication.

What to do if this sounds familiar

  • Start with your doctor: a proper diagnosis needs a blood test, and this should be your first step if you recognise several of the 9 signs.
  • Consider the temperature test as a helpful extra check: especially if your symptoms persist despite a normal blood result.
  • Look at your iodine intake carefully: both too little and too much can be problematic, so this is one to discuss with a qualified practitioner rather than self-prescribe.
  • Think about your wider nutrient intake: selenium, B vitamins, zinc and manganese all play a genuine part in healthy thyroid hormone function.
  • Cook goitrogenic foods well and eat them in moderation rather than cutting them out unnecessarily.

Common questions about an underactive thyroid

Why are women more affected than men? Four times as many women as men are diagnosed with an underactive thyroid, which is why thyroid function is often worth checking when unexplained weight gain, fatigue or menstrual changes appear.

Can my blood test be normal and I still have a problem? It's possible. If your thyroid produces enough hormone but your cells don't respond to it properly, tests can look normal. The temperature test is a useful additional check to raise with your doctor.

Should I take iodine? Not without guidance. Too much iodine can worsen an underactive thyroid, making this one of the few nutrients where more is genuinely not better.

Can thyroid problems affect fertility? Yes, difficulty conceiving is one of the nine signs, and thyroid function is often checked as part of fertility investigations.

The bottom line

An underactive thyroid is far more common in women than men, and its symptoms, gradual weight gain, feeling the cold, low mood, hair changes and fatigue, can easily be mistaken for simply having a tough few months. If several of these 9 signs sound familiar, the sensible next step is a conversation with your doctor, alongside a genuine look at the nutritional factors known to support healthy thyroid hormone function.

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This article is for information only and is not intended to be a substitute for medical advice. Always consult a qualified doctor or health practitioner before making changes to your diet or supplement routine.

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