# Your thyroid and your cycle

> The little butterfly gland in your neck has a big say in your periods.

Category: Conditions · 4 min read · Written by the Splash team · Sources checked October 2026
URL: https://splash.care/learn/conditions/thyroid

## Quick answer

- Thyroid problems are common in women and can change your periods.
- Underactive often means irregular or heavier periods; overactive often means lighter or missed ones.
- A blood test can check, and treatment is usually simple.

Your thyroid makes [hormones](https://splash.care/learn/cycle-basics/hormones-explained) that set the pace for loads of body systems, including your [menstrual cycle](https://splash.care/learn/glossary/menstrual-cycle). When it runs too slow or too fast, your periods often notice.

**5 to 8×** women are five to eight times more likely than men to have a thyroid condition (Cleveland Clinic)

## Underactive vs overactive

- **Underactive** (hypothyroidism): tiredness, feeling cold, weight gain, dry skin and hair, [low mood](https://splash.care/learn/mind-mood/why-mood-shifts), and [irregular](https://splash.care/learn/cycle-basics/irregular-cycles) or [heavy periods](https://splash.care/learn/period-care/heavy-periods)
- **Overactive** (hyperthyroidism): racing heart, trouble sleeping, weight loss, feeling hot and sweaty, [anxiety](https://splash.care/learn/mind-mood/luteal-anxiety) or irritability, and sometimes lighter, less frequent or missing periods

Both can affect fertility, and many symptoms overlap with stress, [PMS](https://splash.care/learn/glossary/pms) or [anaemia](https://splash.care/learn/glossary/anaemia), which is why a simple blood test is so useful.

## Getting checked

A GP can arrange a thyroid function blood test, which usually starts with [TSH](https://splash.care/learn/glossary#tsh). Both conditions are very treatable, usually with daily tablets (for underactive) or medicines that calm thyroid activity (for overactive). NICE heavy-bleeding guidance says thyroid tests are not needed for heavy periods alone unless there are other signs of thyroid disease, so mention any other symptoms.

**Quick quiz:** Which change in periods is more linked to an underactive thyroid?

1. Heavier periods
2. Periods stopping entirely
3. No effect at all

Answer: Heavier periods. The NHS lists irregular or heavy periods with an underactive thyroid. An overactive one is more often linked to lighter, less frequent or missing periods.

If you have an underactive thyroid and are pregnant or planning a pregnancy, tell your GP early. Your dose of levothyroxine will probably need to change, and untreated thyroid problems can raise the chance of complications.

> **Splash says:** if your cycles have changed AND you feel tired, cold or wired for weeks, mention both together. The combo is a clue.

### When to see a doctor

See a GP if you have ongoing symptoms of an under- or overactive thyroid, or your periods change with no obvious reason. Seek urgent help (999) for a very fast or irregular heartbeat with chest pain, severe breathlessness or confusion.

## Sources

1. [NHS: Underactive thyroid (hypothyroidism)](https://www.nhs.uk/conditions/underactive-thyroid-hypothyroidism/)
2. [NHS: Overactive thyroid (hyperthyroidism)](https://www.nhs.uk/conditions/overactive-thyroid-hyperthyroidism/)
3. [Cleveland Clinic: Thyroid disease](https://my.clevelandclinic.org/health/diseases/8541-thyroid-disease)
4. [British Thyroid Foundation](https://www.btf-thyroid.org/)

This is general information, not medical advice. Splash can’t diagnose. If you’re worried, talk to a GP, pharmacist or sexual health clinic. In an emergency call 999 (UK) or 911 (US).