5 conditions made worse by low vitamin D levels

Low vitamin D is rarely the full picture. But when levels drop, it can quietly worsen a range of common health conditions, often through inflammation, hormone signalling, and metabolic pathways.

If you're experiencing symptoms of low vitamin D (such as fatigue or weakness) or have confirmed vitamin D deficiency, these are the conditions where it matters most.

In this article we cover:

1. Osteoporosis (bone health)
2. Cardiovascular conditions (heart health)
3. Type 2 diabetes
4. Polycystic ovary syndrome (PCOS)
5. Thyroid conditions
6. Vitamin D FAQs

1. Osteoporosis (bone health)
 

No matter your age, vitamin D is vital for bone health – without enough of it the body struggles to absorb calcium properly [1]. Over time, this can accelerate loss of bone density and put you at an increased risk of bone fractures.

Symptoms to look out for

Usually, a broken bone is the first sign of osteoporosis, although if you are older, you may experience severe back pain and a hunched posture [2].

What to do

Monitoring your vitamin D levels, eating enough calcium-rich foods, and considering a vitamin D supplement are the best ways to reduce your risk.

2. Cardiovascular conditions (heart health)
 

Low vitamin D doesn’t just affect your bones, it also plays a role in regulating your heart health. If you have a vitamin D deficiency, it may contribute to high blood pressure and inflammation.

Some studies show low vitamin D levels are related to cardiovascular conditions [3].

These include:

• Hypertension
• Stroke
• Heart failure

According to the British Heart Foundation not getting enough vitamin D could cause high blood pressure and also contributes to chronic inflammation [3].

As to whether supplementing with vitamin D can actually reduce the occurrence of cardiovascular events, it’s a mixed bag of results – some studies found it may reduce events while other saw no difference [4,5]. It could be that a vitamin D deficiency in relation to heart conditions is just a side effect of not getting enough exercise outdoors [6].

But if you do have a vitamin D deficiency, there’s no harm in getting some sun on your skin or trying a vitamin D supplement, as per the NHS guidelines.

Symptoms to look out for

Cardiovascular disease often develops gradually. Early signs can include fatigue, shortness of breath during activity, or elevated blood pressure.

What to check

Alongside checking your vitamin D levels, it can be useful to monitor your cholesterol and inflammation markers such as CRP to get a clearer picture of your cardiovascular health.

Top biomarkers to check:

• Hs-CRP – inflammation marker
• Lipoprotein(a) or Lp(a) – genetically determined heart health risk
• Lipid profile – cholesterol and triglycerides

You can read more about the best blood tests for heart health in our blog.

3. Type 2 diabetes
 

Vitamin D deficiency is common in people with type 2 diabetes. The sunshine vitamin is thought to improve our body’s response to insulin (the hormone that regulates blood sugar levels). Therefore, having optimal vitamin D levels reduces the risk of insulin resistance, often a precursor to diabetes [7].

Low levels of Vitamin D are linked to poor blood glucose control, higher inflammation, and a higher risk of developing type 2 diabetes.

Some studies suggest vitamin D supplementation may help avoid developing diabetes from prediabetes, although others suggest this may not be useful.

Symptoms to look out for

Common signs include increased thirst, frequent urination, fatigue, and feeling tired after eating.

What to check

Testing your vitamin D alongside HbA1c, fasting glucose, and fasting insulin can help you understand your metabolic health and identify early signs of insulin resistance or prediabetes.

4. Polycystic ovary syndrome (PCOS)
 

Another condition relating to insulin resistance is PCOS. Although PCOS is primarily a hormonal condition that affects how women’s ovaries work, insulin resistance is a common associated factor.

Not only could vitamin D contribute to effect insulin response, but it can also help with some common symptoms of PCOS such as low mood, androgen balance, and fatigue.

Symptoms to look out for

Irregular periods, acne, excess facial or body hair, and difficulty losing weight are common features of PCOS.

What to do

In addition to vitamin D, testing hormones such as testosterone and sex hormone-binding globulin (SHBG), as well as insulin and HbA1c, can help build a more complete picture of your health with PCOS.

If you’re interested in learning more about the condition, our PCOS guide covers everything from supplements and exercises for PCOS, to how it affects your fertility and common health risks associated with the condition.

5. Thyroid conditions
 

If you have a thyroid condition such as Hashimoto’s or Graves’, you may want to check your vitamin D levels. Vitamin D also plays a role in immune regulation. It’s common to see low vitamin D alongside thyroid dysfunction, especially where antibodies are involved as vitamin D deficiency is linked with increased thyroid antibodies.

Symptoms to look out for

Fatigue, unexplained weight changes, sensitivity to cold, and hair thinning are common signs of a thyroid imbalance.

What to do

If you already know you have a thyroid condition, it might be worth keeping an eye on your vitamin D levels too.

We have a few thyroid blood tests that may be helpful if you haven’t yet been diagnosed with a thyroid condition but suspect one.

You should always check with your healthcare provider before starting supplements. If you want more information about vitamin D supplements, head over to our vitamin D supplement guide.

Vitamin D deficiency FAQs

How do I know if my vitamin D is low?

Low vitamin D doesn’t always cause obvious symptoms, but common signs include fatigue, low mood, muscle weakness, and frequent illness.

A blood test is the only reliable way to confirm if your levels are low.

Can vitamin D deficiency cause hair loss?

Vitamin D deficiency may contribute to hair thinning by affecting hair follicle cycling and immune regulation. It’s often seen alongside other deficiencies such as low iron or B12, although in other cases hair loss may be due to hormonal or autoimmune conditions.

Does low vitamin D cause fatigue?

Low vitamin D is commonly linked to fatigue, likely due to its role in muscle function, inflammation, and energy metabolism. Many people report improved energy levels once deficiency is corrected.

Does vitamin D deficiency cause depression?

Vitamin D deficiency is associated with low mood and depression, as it plays a role in brain function and inflammatory pathways. It’s often one of several contributing factors rather than a sole cause.

How long does it take to fix vitamin D deficiency?

Vitamin D levels typically improve within 8–12 weeks with appropriate supplementation, although this can vary depending on how low levels are and the dose used. A follow-up blood test can be considered to confirm vitamin D levels have normalised.

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References

1. NHS, Vitamin D. In: nhs.uk [Internet]. 23 Oct 2017 [Accessed 2 Apr 2026]. 
2. Osteoporosis. In: nhs.uk [Internet]. 3 Oct 2018 [cited 20 Mar 2026]. 
3. Vitamin D deficiency could cause high blood pressure - BHF. [cited 31 Mar 2026]. 
4. Vitamin D improves heart function in people with chronic heart failure. In: British Heart Foundation [Internet]. [cited 31 Mar 2026]. 
5. Thompson B, Waterhouse M, English DR, McLeod DS, Armstrong BK, Baxter C, et al. Vitamin D supplementation and major cardiovascular events: D-Health randomised controlled trial. BMJ. 2023;381: e075230. doi:10.1136/bmj-2023-075230
6. Trimarco V, Manzi MV, Mancusi C, Strisciuglio T, Fucile I, Fiordelisi A, et al. Insulin Resistance and Vitamin D Deficiency: A Link Beyond the Appearances. Front Cardiovasc Med. 2022;9. doi:10.3389/fcvm.2022.859793
7. Watts M. Vitamin D is a fat-soluble vitamin that plays a number of important roles in the body, including maintaining bone health and assisting immune system function. In: Diabetes [Internet]. 15 Jan 2019 [cited 31 Mar 2026].