Thyroid problems can indeed affect your menstrual period.
Hyperthyroidism (an overactive thyroid gland) is the overproduction of thyroid hormones. Hyperthyroidism can be asymptomatic but it can also affect your menstrual cycle by decreasing the frequency of your periods and making your periods lighter. In severe cases, hyperthyroidism can cause menstruation to cease.
Hypothyroidism (an underactive thyroid gland) is the underproduction of thyroid hormones. Hypothyroidism can also be asymptomatic but it can also affect your menstrual cycle by increasing the frequency of periods, causing heavy periods with increased or prolonged menstrual bleeding, or even increase the pain and discomfort of menstruation.
Since you’ve already had a thyroid lobectomy and are on replacement thyroid hormone (Synthroid), you can suffer from both hyper and hypothyroidism based on your dosage of Synthroid. There are many drugs and foods that can interact and increase or decrease the metabolism of the replacement thyroid hormone. A simple blood test measuring your thyroid stimulating hormone (TSH) levels can determine if your Synthroid dose is adequate. If not, your doctor can adjust your dosage accordingly.
If your thyroid hormone levels are within normal limits, another issue to consider with menstrual infrequency is menopause. Symptoms such as hot flashes, loss of libido, fatigue, and irritability would all point to the diagnosis of menopause. The average age for menopause in modern women is approximately 50-52 years. By definition, a woman is menopausal if she has no menses for a period of 12 months and all other medical reasons have been excluded.