6 Signs You Need Blood Work ASAP, According to Experts

Routine blood work is one of the simplest and most effective ways to catch potential health issues early. Regular screening provides a snapshot of your overall health, offering insights into how well your organs are working, whether your nutrient levels are balanced and if silent conditions like high cholesterol or prediabetes are starting to develop. 

“Lifestyle diseases, such as cardiovascular disease and diabetes, begin with silent symptoms that don’t develop overnight,” says Lauren Harris-Pincus, M.S., RDN. “Regular blood work can monitor changes and trends over time to spot issues before they progress too far,” she adds. 

While some hidden issues can be picked up during a standard checkup, others can develop more rapidly and require urgent attention. We spoke with health experts to share six signs it’s time to get your blood work done ASAP.

1.  Sudden and Unexplained Weight Changes

When your weight suddenly shifts without changes in diet, exercise or lifestyle, it may be a red flag that warrants prompt attention.

Rapid, unexpected weight gain can signal thyroid or hormonal imbalances, or fluid retention linked to heart, kidney or liver issues. For instance, sudden extreme swelling in the lower legs may be a sign of heart failure. A comprehensive medical evaluation by your health care provider—including blood tests to check thyroid function, blood sugar and other key markers—can help determine whether your weight changes are associated with conditions such as heart disease, diabetes, hypothyroidism or something else. 

On the other hand, sudden weight loss—defined as unintentional weight loss of 5% of body weight or more within 6 to 12 months—may indicate metabolic, endocrine or gastrointestinal problems. If this is accompanied by symptoms like extreme fatigue, loss of appetite or an increase in bowel movements or diarrhea, it’s especially important to seek medical attention as soon as possible.

A conversation with your health care provider will help determine which blood tests are needed to identify the underlying cause of sudden, unexplained weight changes.

2. Extreme Fatigue

It’s normal to feel tired from lack of sleep, stress or a short-term illness. However, if you’re regularly experiencing extreme fatigue despite a balanced lifestyle, it may be time to get your blood work checked.

Persistent fatigue can be a sign of anemia, which can be detected with a complete blood count (CBC) and tests that measure iron levels, explains Jennifer L. Armendariz, RN, FNP-C, AOCNP. Iron helps transport oxygen throughout the body, and when iron levels are low, your cells don’t receive enough oxygen, leading to low energy and fatigue. Other nutrient deficiencies, such as vitamin B12 deficiency, may also play a role.

Fatigue may also stem from hypothyroidism, a condition where your thyroid does not produce enough hormones to regulate metabolism, energy levels and other bodily functions., Your health care provider will usually start by checking thyroid-stimulating hormone (TSH), triiodothyronine (T3) and thyroxine (T4) levels to assess thyroid function.

Finally, extreme fatigue can sometimes be linked to more serious conditions such as certain cancers, autoimmune disorders, heart disease, diabetes or sleep apnea. Because these causes can vary, it’s important to work closely with your health care provider for a well-rounded assessment.

3. Excessive Thirst and Frequent Urination

Excessive thirst and frequent urination are hallmark symptoms of high blood sugar. When blood glucose levels rise, the body attempts to remove the excess sugar through urine. This increased urination leads to water loss, which in turn triggers thirst mechanisms to replace lost fluids—creating a continuous cycle.  

This pattern of frequent urination and persistent thirst is a classic sign of diabetes. Your health care provider may order blood tests such as fasting plasma glucose, hemoglobin A1c (HbA1c)—which reflects your average blood sugar over three months—or an oral-glucose tolerance test to confirm the diagnosis.

Although excessive thirst or frequent urination are often associated with diabetes, it can also be a sign of other issues, such as prostate problems, notes Armendariz. In such cases, your health care provider may recommend testing prostate specific antigen (PSA) levels. 

4. Easily Bleeding or Bruising

If you bruise or bleed easily, your health care provider may order blood tests such as a complete blood count (CBC) and clotting studies to rule out a platelet disorder, says Cynthia Odogwu, M.D. Platelet disorders can affect how blood flows throughout the body, potentially leading to excessive bleeding or, conversely, an increased risk of blood clots. Genetics, lifestyle habits like alcohol or tobacco use, certain medications or underlying medical conditions may all increase your risk.

In addition to easy bruising and bleeding, platelet-related issues can cause symptoms such as extreme fatigue, heavy menstrual cycles, blood in the stool or urine, frequent gum or nosebleeds and small red patches on the skin known as petechiae.

Armendariz adds that she would also recommend an iron panel when there is excessive or abnormal bleeding, since significant blood loss can deplete iron levels. “Typically, an iron profile includes ferritin (a stored iron level) [which] will let the health care provider know if someone is iron-deficient and if the patient needs iron replacement,” she explains.

5. Hair Loss

Although short-term illness or ongoing stress can sometimes lead to temporary hair loss, it may also signal an underlying health issue. Two common causes are hormonal imbalances and low iron levels.

Iron plays a vital role in the hair growth cycle, with low levels leading to increased, premature shedding. In some cases, poor overall nutrition—such as inadequate intake of calories, protein, vitamin D or zinc—can also contribute to hair loss or thinning. Your health care provider may order bloodwork to check for iron deficiency and other vitamin and mineral deficiencies. They may also recommend a visit with a registered dietitian to discuss your diet and eating habits.

Thyroid hormones are also essential for hair growth and maintenance, with hair loss potentially indicating a thyroid disorder. With hyperthyroidism (an overactive thyroid) your hair may shed quickly or fall out in clumps. On the other hand, hypothyroidism (an underactive thyroid) may present as slow hair growth, general hair thinning or loss of the outer eyebrows. To be sure, your health care provider may test thyroid-stimulating hormone (TSH) levels to help identify whether thyroid function is too high or low.

6. Feeling Hot and Cold

If you often experience temperature changes that others don’t—such as feeling unusually cold or hot—it may be time to check your blood count and iron levels, says Odogwu. 

Feeling persistently cold, especially in your hands and feet, can be a sign of iron-deficiency anemia—a condition in which the body doesn’t have enough iron to produce hemoglobin, which limits oxygen delivery and may cause your extremities to feel cold.

On the other hand, regularly feeling overheated may be a sign of hormonal imbalances linked to conditions such as menopause or hyperthyroidism. Blood tests that measure thyroid levels such as TSH, free T3, free T4, luteinizing hormone (LH), estrogen (estradiol) and follicle stimulating hormone (FSH) can help determine whether these factors are contributing.

When Blood Tests Are Especially Important

Ideally, try to get blood work done at least once a year, during a preventive health visit or physical, says Odogwu. 

Beyond regular checkups, there are instances that may require additional blood work, such as:

• Medication Use. Certain medications affect the absorption of nutrients. For example, metformin affects vitamin B12, so it is essential to regularly check vitamin B12 levels when taking this medication to prevent deficiency.
• Starting a New Medication. Armendariz recommends getting blood work when you start a new medication, such as thyroid, diabetes or cholesterol medications. “Blood work will typically be done about three months after starting a medication to check how effective it is,” Armendariz explains.
• If You’re on a GLP-1. “When overall food intake is low, it can be challenging to consume everything you need without supplements,” notes Harris-Pincus. In particular, individuals taking a GLP-1 medication should monitor their iron, B12 and vitamin D levels, she explains.
• Supplement Use. Both Odogwu and Harris-Pincus recommend having blood work done before starting and while taking vitamin, mineral or herbal supplements to make sure levels don’t exceed the normal range or affect organ function.
• If You’re at Risk of Health Issues. If you have a family history or are at an increased risk of certain health conditions like heart disease or diabetes, early screening is important, says Armendariz. 

Our Expert Take

Unplanned weight loss or gain, extreme fatigue, hair loss and excessive thirst are just a few signs it may be time to get blood work and talk to a health care professional. Routine blood tests, paying close attention to your body’s signals and open communication with your health care team can help detect potential issues early and support long-term health.