Iron, tiredness and heavy periods: the blood test to ask for

SASian Anderson, Writer and editor, MyGLP. Sources checked 21 October 2026.

Why women run short

A period costs iron every month, and a heavy period costs a lot. The NHS iron deficiency anaemia page names heavy periods and pregnancy as the main causes in women. On a weight loss medicine you eat less, and often less red meat in particular, so the iron coming in drops at the same time. The result is the most common nutrient deficiency in UK women, and most of the women who have it do not know.

The symptoms

• Tiredness and lack of energy.
• Shortness of breath on stairs or a hill.
• Heart pounding or racing.
• Pale skin, or pale inside the lower eyelid.
• Headaches.
• Hair shedding, brittle nails, sore tongue, cracks at the corners of the mouth.
• Restless legs at night.
• Craving ice or non-food things.

Every one of those overlaps with the side effect list of a weight loss medicine, with the perimenopause, and with an underactive thyroid. That is why the test matters. Guessing gets you nowhere.

The test

Ask your GP for a full blood count and a ferritin level. The full blood count shows whether you are anaemic, meaning your haemoglobin is low. Ferritin shows how much iron is in store, and it drops long before haemoglobin does. You can be short of iron, and feel it, with a normal haemoglobin. NICE guidance treats a ferritin below 30 micrograms per litre as iron deficiency in adults. Many labs print a normal range that starts at 12 or 15, so a result of 20 can come back marked normal while you feel awful. Ask for the number, not the word.

Ferritin also rises with inflammation and infection, so a GP may check a marker called CRP alongside it. Tell the GP about heavy periods, any gut symptoms and your diet, so the cause gets looked at as well as the level.

Food

Vitamin C doubles the absorption of plant iron. Orange juice with a fortified cereal, peppers with lentils, a kiwi after a tofu stir-fry. Tea and coffee cut absorption, so keep them an hour away from iron-rich meals. Calcium does the same, which means a milky coffee with your spinach omelette undoes some of the good.

Tablets

If ferritin is low, the GP will usually prescribe iron tablets. The old advice was three a day. Research in the last few years30182-5/fulltext) showed that one tablet on alternate days is absorbed better than daily dosing, because a big dose switches off absorption for the next day. Many GPs now prescribe one tablet every other day, which also cuts the constipation and nausea that put people off. Take it with orange juice, away from tea, coffee, milk and your weight loss medicine's worst nausea hours. Stools go black, which is normal. Expect to stay on them for three months after the level recovers, to rebuild the store.

If tablets do not suit your stomach, which is common on a weight loss medicine, say so. Liquid iron, a different salt, or an iron infusion at the hospital are all options. Supermarket multivitamins hold too little iron to fix a deficiency.

Fixing the cause

Replacing iron while a heavy period removes it every month is bailing a boat with a hole in it. The NHS heavy periods page lists the treatments, from tranexamic acid tablets taken on heavy days to the hormonal coil, which cuts bleeding by up to 90% for many women. Ask about them in the same appointment.

Sources

1. Iron deficiency anaemia (NHS)
2. Anaemia, iron deficiency: clinical knowledge summary (NICE)
3. Iron absorption from oral iron supplements given on consecutive versus alternate days (Stoffel et al, The Lancet Haematology, 2017)
4. Heavy periods (NHS)
5. Iron: vitamins and minerals (NHS)