For Goldilocks, her porridge had to be "just right". Similarly, the thyroid's production of hormones has to be just right. Otherwise, the body's metabolism may be upset.
The thyroid, a butterfly-shaped gland at the front of the neck - near the Adam's apple in men - makes and releases two essential hormones, T4 (thyroxine) and T3 (triiodothyronine).
The numbers "4" and "3" refer to the number of iodine atoms in each molecule of the hormone. For example, T4 has four iodine atoms.
Thyroid hormones control metabolism, or the rate at which every part of the body works.
While thyroid disorders range from a small, harmless goitre (enlarged gland) to cancer, the most common problems involve an abnormal production of thyroid hormones.
Over-production may result in hyperthyroidism while under-production can lead to hypothyroidism.
In hyperthyroidism, the body is revved up like a race car. Symptoms include weight loss, increased appetite, intolerance to heat, irritability and rapid heartbeats.
Women are four to five times more likely to develop hyperthyroidism, especially those aged between 20 and 50, said Dr Peter Eng, a consultant endocrinologist at Peter Eng Endocrine Clinic.
Professor Lee Kok Onn, the head of the division of endocrinology at National University Hospital, said the most common cause of hyperthyroidism - and all thyroid disorders here - is Graves' disease.
In Grave's disease, the body's immune system produces abnormal antibodies which stimulate the thyroid gland, causing an over-production of hormones.
Left untreated, hyperthyroidism can increase one's risk of an irregular heart beat and osteoporosis, said Dr Eng.
There are three treatment methods: anti-thyroid drugs, radioactive iodine and surgery.
Anti-thyroid drugs, usually taken for up to two years, suppress thyroid hormone production.
Radioactive iodine (taken orally) and surgery destroy or remove most of the thyroid gland, thus reducing the amount of thyroid hormones produced.
Prof Lee said that radioactive or surgical treatment for hyperthyroidism may lead to an opposite effect - hypothyroidism.
As the body starts to slow down, hypothyroidism symptoms include weight gain, depression, constipation and feeling cold and tired.
Hypothyroidism, which has been found to be more common in Indians here, can also be caused by an autoimmune response. Called Hashimoto's Thyroiditis, the condition causes the immune system to attack the thyroid gland, causing it to become underactive.
Most patients need to be on thyroid hormone supplements - usually lifelong - to make up for the deficiency.
Dr Daphne Khoo, the head and senior consultant at the department of endocrinology at Singapore General Hospital, said that Graves' Disease and Hashimoto's Thyroiditis may also cause the gland to enlarge abnormally.
Called a goitre, the enlargement can also be caused by many large lumps, or nodules, in the gland. In this case, it is called a multi-nodular goitre.
The enlargement is often visible, but a medical examination is needed when it is less obvious, she said.
Treatment of the goitre is based on its underlying condition.
If it is caused by Hashimoto's Thyroiditis, thyroid hormone supplements are given. For Graves' Disease, anti-thyroid drugs, radioactive iodine or surgery are the options.
For multi-nodular goitres, no treatment may be necessary if these three conditions are met: the nodules are found to be non-cancerous; thyroid hormone levels are normal; and the enlargement does not compress on other neck structures.
In most cases, the goitre shrinks with treatment. It is uncommon for multi-nodular goitres to shrink spontaneously, said Dr Khoo.
Cancer can also develop in the thyroid gland.
Thyroid cancer is the 10th most common cancer in women here. It does not rank within the top 10 cancers for men.
However, Dr Ranjiv Sivanandan, the director at the Thyroid, Head & Neck Surgery Centre, said large US studies show that its incidence in both sexes is increasing at a rate of 6 to 7 per cent a year, faster than any other cancer.
The most common symptom of thyroid cancer is a painless nodule in the gland. However, he said that 90 to 95 per cent of gland nodules are benign.
The challenge, said Dr Ranjiv, is to identify the malignant nodules from the multitude of benign ones.
Nodules that are accompanied by voice hoarseness, rapid growth and other neck lumps should raise the suspicion of cancer. Diagnosis is usually confirmed through a thyroid ultrasound and biopsy.
Thyroid cancer is seldom mistaken for other thyroid conditions, particularly those related to thyroid hormone production.
The condition is treatable, said Dr Ranjiv. Surgery is usually the first step, where the affected part of the thyroid and lymph nodes are removed. Recovery is quick, with little side effects.
Radioactive iodine is often given next to remove remnant cancer cells.
Patients are then put on thyroxine suppression therapy to replace the lost thyroid hormones. It also helps to suppress TSH, or thyroid stimulating hormones. TSH usually trigger production of thyroid hormones but can also stimulate thyroid cancer cells in cancer patients.
The prognosis for thyroid cancer is bright. Said Dr Ranjiv: "Early diagnosis and treatment of thyroid cancers in young patients with tumours less than 4cm result in a cure rate of up to 99 per cent.
"It is a prognosis that far surpasses any other solid organ cancer."