Weight gain and a late period - is it PCOS or thyroid?

Author: Aunt Vadge
Qualified Naturopath | BHSc(N)

Last reviewed: July 7, 2026

Dear Aunt Vadge,

I’ve recently gained a lot of weight, and this time I haven’t got my period – it’s now 20 days past when it was due. I thought my weight might be the reason, so I wanted to ask. What should I do?

Sincerely,
Missing Period
India

Dear Missing Period,

You’ve actually spotted something clever, so I’ll just flip it round, because the connection probably runs the other way. Rather than the weight causing the missed period, it’s very common for one hormonal shift to cause both at once – the weight change and the cycle change being two symptoms of the same thing.

Which means the useful question isn’t ‘how do I fix the weight’, it’s ‘what’s nudged my hormones?’

Two things are worth checking. The first is PCOS, whose two hallmark features are exactly yours: irregular or absent periods plus weight that creeps on (it’s driven by blood-sugar and insulin dysregulation, not ‘cysts’, despite the name). Have a read and see if other bits ring true – acne, extra hair growth, sugar cravings.

The second is your thyroid: an underactive thyroid is another classic cause of weight gain and missed periods together, along with tiredness and feeling the cold, and it’s a simple blood test worth asking for.

So please don’t cast weight as the villain – it’s more likely a clue pointing at your hormones, and that’s the thing to investigate, kindly rather than punishingly.

As for the late period itself: your period runs like clockwork from the day you ovulate, about two weeks later, so a ‘late period’ really means late or skipped ovulation. Ovulation is flexible and shifts a bit each cycle – stress, travel and illness can all push it back.

A one-off long cycle or a single skipped month is usually nothing. But a cycle stretching past 35 days, especially if it keeps happening, is worth attention, and that’s exactly the pattern that points at PCOS or thyroid.

So start tracking your cycle and noting your symptoms, and if your period stays away another month or this becomes a habit, see a doctor and ask specifically about why periods stop, PCOS and thyroid included. You’re asking exactly the right questions.

Warmest regards,
Aunt Vadge

This is general information, not a substitute for personalised medical advice.