"I'm tired all the time" is one of the most common things teenagers say, and most of the time it reflects late nights, school pressure, training or a growth spurt. Sometimes, though, a parent, coach or therapist wonders whether something in the blood is part of the picture. Teen blood tests for iron, vitamin D and thyroid are the three that come up most often. This guide explains what each one measures and when testing tends to make sense.
Why iron comes up so often in teen blood tests
The teenage years combine rapid growth, the start of periods for many girls and, for some, a heavy training load. A review on heavy menstrual bleeding describes it as a major contributor to iron deficiency in girls and women from the first period onwards, and notes that the problem is often treated as normal by families and professionals alike (Munro et al., 2023). The NHS lists feeling tired or short of breath a lot among the signs of heavy periods.
It is worth keeping a sense of proportion. A 2026 study of 485 girls aged 15 to 19 in Sweden found that iron status on its own was not linked to how tired they felt; heavy periods, skipping breakfast or lunch and daily nicotine use were (Stubbendorff et al., 2026). In other words, a blood test can answer one question, but tiredness usually has more than one cause.
Iron Status Profile and Ferritin
The Iron Status Profile measures five markers: serum iron, ferritin, transferrin saturation, total iron-binding capacity and unsaturated iron-binding capacity. Together they show how much iron is circulating now and how full the body's stores are. The LML explains that ferritin is the storage protein, so a low value means low reserves. It also notes that ferritin can be raised by inflammation, infection or alcohol, which is why it is best read together with the other markers.
Ferritin is also available as a single test. It can be a reasonable choice for follow-up once a fuller iron picture already exists.
Vitamin D
Vitamin D helps the body manage calcium and phosphate, which matter for bones, teeth and muscles. The NHS explains that between October and early March the sun in the UK is not strong enough for the body to make enough vitamin D, and advises everyone to consider a daily supplement of 10 micrograms in autumn and winter. The LML notes that low levels are common at the end of winter in the UK. Teenagers who train indoors, cover most of their skin or spend little time outside are the ones for whom the question comes up most often.
Thyroid Stimulating Hormone (TSH)
TSH is made by the pituitary gland and tells the thyroid how much hormone to produce. The LML describes it as the test used to check and monitor thyroid function, for both an underactive and an overactive thyroid. A review on hypothyroidism notes that its onset is usually gradual and that the way it shows up varies a lot, especially in children (Chaker et al., 2022). That is why a TSH result is interpreted alongside how the young person is feeling, not in isolation.
