An underactive thyroid (hypothyroidism) develops when your thyroid makes too little hormone. Your system shifts down a gear: you get tired, feel cold and sometimes gain weight without eating differently. In women the most common cause is Hashimoto's autoimmune disease (Chaker et al., 2017). Two blood values, a raised TSH and a lowered free T4, usually confirm the picture.
Many women live with it for months before linking the complaints to their thyroid. The symptoms are creeping and resemble so many other things. Below you will read what to recognise, how an underactive thyroid shows up in your blood, and which next step makes sense.
What is an underactive thyroid?
In an underactive thyroid the gland produces less T4 and T3 than your body asks for. Your brain responds by making more TSH, trying to push the thyroid along. That is why hypothyroidism shows a high TSH alongside a normal or low free T4.
The best-known form is subclinical hypothyroidism: your TSH is mildly raised while your free T4 is still normal. Whether that causes complaints differs per person. You get a fuller picture by reading the values together, not in isolation. See our overview of what your thyroid values mean.
Underactive thyroid symptoms in women
The symptoms come from a slowed metabolism. Everything runs slower: energy, digestion, heart rate and thinking. Women often recognise a mix of fatigue, feeling cold and weight gain that does not match their lifestyle.
- Fatigue that does not lift with rest
- Feeling cold quickly, especially hands and feet
- Weight gain or difficulty losing weight
- Dry skin, brittle hair and hair loss
- Constipation and slow digestion
- A low mood and slow, foggy thinking
- A heavier or irregular period
That last point matters for women: an underactive thyroid can affect your cycle and your fertility. If you wish to conceive, the thyroid is well worth including. For hair complaints, see our article on hair loss and which hormones to test.
What does an underactive thyroid look like in your blood?
The combination of values tells the story. With an underactive thyroid the pattern points one way: TSH up, free T4 down or still normal. Anti-TPO shows whether an autoimmune cause sits behind it.
| Value | With an underactive thyroid | What it means |
|---|---|---|
| TSH | Raised | Your brain is asking for more hormone |
| Free T4 | Low or normal | The thyroid delivers too little (or just enough) |
| Anti-TPO | Often raised | Points to Hashimoto as the cause |
A mildly raised TSH with a normal free T4 is called subclinical. A strongly raised TSH with a low free T4 fits clear hypothyroidism. Which way your result points is something to discuss with your doctor.
What is the difference with an overactive thyroid?
An underactive thyroid slows down, an overactive one speeds up. Where you are tired and cold, someone with an overactive thyroid has palpitations, loses weight and feels restless. The blood values are mirror images. If you are unsure which way your complaints point, read about the symptoms of an overactive thyroid.
When should you test for an underactive thyroid?
Testing is worthwhile if your complaints persist, or if thyroid disease runs in your family. A single measurement of TSH and free T4 quickly gives clarity. At Lunara the Thyroid Function measures these two core values, without a GP referral.
If you also want to know whether Hashimoto is involved, anti-TPO adds insight. Persistent fatigue, by the way, is not always about your thyroid; read also when fatigue is and is not caused by your thyroid. This whole topic sits under our overview of thyroid symptoms in women.
Frequently asked questions
Can an underactive thyroid resolve on its own?
A lasting hypothyroidism caused by Hashimoto usually does not resolve on its own. A temporary form, for example after pregnancy, can recover. A repeat measurement helps tell them apart; discuss this with your GP.
Does an underactive thyroid make you gain weight?
An underactive thyroid can encourage weight gain because your metabolism slows. It is often a few kilos, partly fluid. Large weight swings usually have other causes too.
What time of day should I test TSH?
TSH sits slightly higher in the morning than later in the day. For a reliable comparison between measurements, test at roughly the same morning time each time.
References
- NHG-Standaard Schildklieraandoeningen (M31). Nederlands Huisartsen Genootschap. Available via nhg.org.
- Chaker L, Bianco AC, Jonklaas J, Peeters RP. Hypothyroidism. Lancet. 2017;390(10101):1550-1562. PMID: 28336049.
- Taylor PN, Albrecht D, Scholz A, et al. Global epidemiology of hyperthyroidism and hypothyroidism. Nat Rev Endocrinol. 2018;14(5):301-316. PMID: 29569621.
Every blood test result through Lunara includes a professional assessment by a BIG-registered doctor. For treatment decisions, discuss your results with your GP.
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