Autumn—the season of falling leaves, wooly socks  and… dwindling Vitamin D levels. If you have Hashimoto’s or hypothyroidism, now is the time to check your Vitamin D status before winter fully sets in.

Your Vitamin D levels should be at their highest at the end of summer, thanks to all those magical ultraviolet rays. But as the days grow shorter & darker, your stores will naturally decline. If your levels are below optimal now then by the time winter is in full swing, you may become deficient at a time when it is difficult to correct—and that’s not great news for autoimmunity.

Why Vitamin D Matters for Hashimoto’s

• Immune Regulation: Vitamin D is essential for regulating both the innate and adaptive immune system. It supports the function of T-regulatory cells, which help prevent the immune system from attacking your own tissues—including your thyroid. It also helps reduce the production of inflammatory cytokines, which are often elevated in autoimmune conditions like Hashimoto’s. In short, adequate vitamin D stores are really important if you have Hashimoto’s. Some studies have even shown that correcting vitamin D stores has a positive effect on thyroid antibody levels.
• Thyroid Function: People with hypothyroidism and Hashimoto’s are more likely to be deficient in Vitamin D. Due to D’s role in T-regulatory cells and autoimmunity prevention it’s likely to be part of the reason autoimmunity develops for some people. Maintaining optimal Vitamin D is an important part of supporting thyroid health.

What is Optimal?

If you have autoimmunity, aim for a blood level close to 150 nmol/L but at least over 100 nmol/L. Lab reference ranges indicate that levels over 50 nmol/L are adequate. These ranges are based on the average person, not the optimal range for someone with an autoimmune condition. And “average” isn’t necessarily the target if you’re trying to feel your best.

Who’s at Higher Risk for Deficiency?

You may be more prone to low Vitamin D if you:

• Have had your gallbladder removed
• Have had bariatric surgery
• Are living with obesity
• Have Inflammatory Bowel Disease or liver disease
• Spend most of your time indoors
• Have darker skin (melanin reduces the skin’s ability to produce Vitamin D from sunlight)
• Take statins (they interfere with Vitamin D production)
• Live far from the equator (lower UV exposure)
• Wear high SPF sunscreen every time you go outdoors

Symptoms of Deficiency

• Fatigue
• Joint pain
• Low mood or seasonal affective disorder (SAD)
• Frequent colds or infections

How to Get Enough Vitamin D

Food Sources (but don’t get too excited…)

Only about 10% of your Vitamin D comes from food. Fatty fish, egg yolks, and fortified foods can help, but they won’t cut it alone.

Sun Exposure

In summer, 10 minutes of midday sun on your upper body (without sunscreen) is usually enough. You need to be sensible here, if it’s a 40+ degree day or an extreme UV rating then that may not be the best time to sit in the sun. You do not want to get burnt. For most skin types 10minutes is a safe length of time to sit in the sun on a moderate UV rated day. But by this time of year? Depending on where you live, the sun may no longer be strong enough to make a real difference.

Supplements

If your levels are below 100 nmol/L you could consider supplementation. If they are below 75 nmol/L I highly recommend you consider it. Personally, knowing what I do about vitamin D and autoimmunity I want levels as close to 150 nmol/L as possible. A few things to keep in mind:

• Choose Vitamin D3 (cholecalciferol), not D2 (D2 is cheaper and less bioavailable—you’ll find it in cheap pharmacy & supermarket supplements and sometimes fortified foods & drinks).
• Vitamin D is fat soluble, so always take it with a meal containing fats (which, if you’ve been following me, should be every meal and snack!).
• You need bile to absorb fat-soluble vitamins like D3. Bile is released when you eat fats, so skipping fats = poor absorption.

Chemists and health food stores generally stock a couple of good Vitamin D3 supplements options. I like Eagle Vitamin D Spray and Bioceuticals D3 Drops Forte as they both provide 1000IU per dose and are easily sprayed or dropped directly into the mouth. They enable flexible dosing too as 1000IU is usually more of a maintenance dose rather than a dose you would use to raise Vitamin D status in someone with a deficiency. I can’t provide dosing advice without a pathology review but generally speaking doses of 2000-5000IU daily are required for a temporary period of time to reach adequate levels. Your dose will vary based on your pathology results and health history. Sometimes GP’s will recommend a single large dose in Autumn or a series of larger weekly doses. Never take fat soluble vitamins like vitamin D long term without testing as they are stored long term and can reach toxic levels in your body.

The Bottom Line

If you have Hashimoto’s, don’t guess—test. Now is the perfect time to check your Vitamin D levels so you can address any deficiencies before winter. Supporting your Vitamin D status is a small but powerful step in managing your thyroid health and supporting your immune system.