Thyroid and your period: how your thyroid shapes your cycle

Your thyroid and your period are more closely linked than most women realise. Thyroid hormone affects your ovaries, your ovulation and the build-up of your womb lining. An underactive thyroid more often goes with heavier and longer periods, while an overactive thyroid can produce shorter and lighter bleeding.

What strikes me most here: search for an irregular cycle in the Netherlands and you land on oestrogen, progesterone and PCOS. The thyroid rarely appears, even though one tube of blood can rule it out.

That is a missed opportunity.

Can your thyroid affect your period?

Yes, and the effect runs along several routes at once. Thyroid hormone contributes to follicle maturation, to ovulation, and to the production of SHBG, the protein that binds your sex hormones. Change thyroid function and you shift the amount of free oestrogen and the build-up of your lining with it.

There is an indirect route too. An underactive thyroid can raise your prolactin, and raised prolactin can suppress ovulation. That is one of the ways a thyroid problem can bring a cycle to a stop.

Research by Krassas and colleagues in women with an underactive thyroid found menstrual disturbances more often than in women without a thyroid problem, with scanty or irregular bleeding and heavier bleeding among the patterns most reported (Krassas et al., 1999).

In a later review the same group described how both an underactive and an overactive thyroid can disturb the cycle (Krassas, 2000).

Which cycle changes go with which thyroid problem?

Roughly: slow means heavier and longer, fast means lighter and shorter. With an underactive thyroid, heavier bleeding and an irregular cycle are the best known patterns. With an overactive thyroid you more often see shorter, lighter periods, and sometimes periods stop altogether.

That table is deliberately not absolute. The pattern gives a direction, not a diagnosis, and plenty of women fit neither column neatly.

If heavy bleeding is what you recognise, heavy periods is the next article. For an erratic cycle it is irregular periods, and if your period stops, a missing period.

Why do periods get heavier with an underactive thyroid?

With an underactive thyroid the build-up and shedding of your womb lining becomes less coordinated. Some cycles run without ovulation, so the progesterone phase that normally limits the bleed is missing. The lining then keeps building and eventually sheds more heavily.

There is a second layer that rarely gets mentioned: heavier bleeding costs iron, and low ferritin produces the tiredness you then attribute to your thyroid.

That creates a loop. The slow thyroid makes your bleeding heavier, the heavy bleeding pulls your iron down, and low iron makes you tired.

Make it concrete: your TSH is 6.8 mIU/L, your period lasts seven days, and your ferritin sits at 9 micrograms per litre. Address the thyroid alone and your TSH comes neatly back down. Your ferritin stays at 9. So you stay tired, while your report looks better than you feel.

I think that is the most useful insight in this article, and it is exactly what a testing brand can see that a hair clinic or a fertility blog cannot. More on it in anaemia from heavy periods and iron deficiency without anaemia.

Can a thyroid problem affect your fertility?

Thyroid hormone plays a role in ovulation and in early pregnancy, so a thyroid that is out of balance can affect the chance of conceiving. Krassas described how both an underactive and an overactive thyroid go along with cycle disturbances, and cycles without ovulation are the most relevant of those (Krassas, 2000).

That is not a reason to panic. It is a reason to include your thyroid in the picture when your cycle changes and you are trying to conceive.

On the period around and after pregnancy we wrote thyroid and pregnancy and thyroid after childbirth.

Is it your thyroid or is it PCOS?

These two get confused more often than you would expect, because both produce an irregular cycle. The difference sits in the accompanying symptoms. With PCOS you more often see acne, excess hair growth and signs of insulin resistance, while with an underactive thyroid, feeling cold, constipation and dry skin come to the front.

One detail makes it more confusing still: an underactive thyroid lowers SHBG, which can push your free testosterone higher. That resembles a PCOS-like picture while the cause sits elsewhere.

Which is why TSH sits as standard in the list of values when working out an irregular cycle. What gets checked in PCOS is in PCOS testing, and on SHBG we wrote SHBG in women.

Which values give information here?

For the thyroid part it comes down to TSH, free T4 and anti-TPO. Alongside that, ferritin is almost always relevant with heavier bleeding, and prolactin when your period stops. Which values fit depends on the pattern you recognise.

Timing matters here too. TSH and free T4 can be measured on any day of your cycle, while sex hormones such as progesterone are tied to a specific cycle phase.

That difference saves time. You do not need to wait for day three of your cycle to check your thyroid values.

Per marker you can read more on our pages for TSH, free T4 and ferritin. The full explanation of the thyroid values is in thyroid values explained.

What to discuss with your GP

Track two or three cycles before you go: cycle length, days of bleeding, how often you change on the heaviest day, and whether there are days you leak through. That makes the word heavy measurable rather than impressionistic.

Mention the symptoms that seem to have nothing to do with your cycle as well. Feeling cold, constipation and dry skin are exactly the clues that make a GP look towards your thyroid.

If you want a baseline of your own, our Thyroid Complete looks at TSH, free T4, free T3 and anti-TPO, and Iron Status at your iron stores. The background to this cluster is in our article on Hashimoto, and on medication in levothyroxine side effects.

Every blood test result includes a professional assessment from a BIG-registered doctor. For treatment decisions, discuss your results with your GP.

References

• Krassas GE, et al. Disturbances of menstruation in hypothyroidism. Clinical Endocrinology (Oxford), 1999. PMID 10468932
• Krassas GE. Thyroid disease and female reproduction. Fertility and Sterility, 2000. PMID 11119728
• Ralli M, et al. Hashimoto's thyroiditis: An update on pathogenic mechanisms, diagnostic protocols, therapeutic strategies, and potential malignant transformation. Autoimmunity Reviews, 2020. PMID 32805423
• NHG guidance on thyroid conditions, Dutch College of General Practitioners
• Thuisarts.nl, patient information on heavy bleeding and the thyroid