Understanding Thyroid function and regulation of Thyroid Hormones which perform important functions in our body helps in the timely diagnosis of Thyroid problems.
Let us start with understanding some terms –
Gland – They are organs of our body that make and release chemical substances, which perform specified body functions. These substances can be enzymes, hormones, substances like sweat, oils, saliva, and other chemical messengers or mediators.
Types of Glands:
Exocrine glands – The substances produced by these glands are released through connected tubes or ducts into other organs, body spaces or on the body surface. Examples include glands of digestion, sweat and oil glands in the skin, salivary glands, etc.
Endocrine glands – These glands produce substances called Hormones which are released directly into the blood. The hormones then reach various other organs in our body to exert their effects and perform specific functions in these organs.
It is the largest endocrine gland of our body situated in the neck below the adam’s apple, which produces and releases thyroid hormones: T4 (called Thyroxine – makes up 90% of thyroid hormones) and T3 (called Triiodothyronine- makes up 10%). Iodine is a key element used in making the thyroid hormones. The thyroid hormones play an important part in metabolism, temperature regulation, growth, and development of the body.
Another endocrine gland called the Pituitary gland (often called the master endocrine gland), at the base of the brain gives out a hormone called TSH (Thyroid Stimulating Hormone) which acts on the Thyroid gland to increase the production of T4 and T3 thyroid hormones. Therefore, if the Thyroid gland is producing fewer thyroid hormones, a feedback signal will go the Pituitary gland to put out more TSH and vice versa.
In a lab test for testing thyroid function –
- High TSH and low T4, T3 is characteristic of reduced functioning (Hypothyroid)
- Low TSH with high T4, T3 suggests over-functioning (Hyperthyroid).
- Normal thyroid hormone levels with TSH being high or low are said to be ‘subclinical’ Hypo or Hyperthyroid respectively.
- Normal Thyroid hormones, as well as TSH levels, is said to be Euthyroid.
Thyroid hormones may be bound to proteins in the blood or be free. It is the free T4 and T3 which exert the actions, therefore, testing TSH along with free T4 (FT4) can give a provisional diagnosis of the Thyroid functional state.
When to suspect and investigate for a thyroid gland dysfunction –suggestive signs and symptoms
1 in 8-10 women can develop thyroid disorders, which is 5 times more risk than men
What is Goiter?
An enlarged or swollen thyroid gland is called goiter which can present with either hypo, hyper or euthyroid features. It can be seen as a neck swelling and may present symptoms like hoarseness of voice or cough, and difficulty in swallowing or breathlessness, as the voice box, food-pipe and wind-pipe are in the vicinity of the thyroid gland.
Causes include iodine deficiency, inflammation (called thyroiditis– seen with infections, ‘autoimmunity’, injury or some drug reactions), and nodules or tumors. Slight enlargement of the thyroid gland is sometimes seen in pregnancy.
Some related terms
Autoimmunity is when our body’s immune system attacks our own body parts, as it by error considers some component of the organ (in this case the thyroid gland) as ‘foreign’. Autoimmune diseases of the thyroid gland can present as either as hyperthyroidism (Grave’s disease) or hypothyroidism (Hashimoto’s disease). Grave’s disease has a typical appearance with retracted eyelids and bulging eyeballs.
Cretinism is a term referring to children whose physical and mental growth is stunted due to being born with a defective and under-functioning thyroid gland.
Myxedema is a term for severe hypothyroidism. In very severe cases, it can result in a ‘myxedema crisis’ which can cause swelling all over the body, fall in BP and body temperature, drowsiness, confusion, shock, seizures, and even coma.
Thyrotoxicosis is a term used for a highly overactive thyroid gland releasing high levels of thyroid hormones T4/T3. In some cases, when thyroid hormone levels go so high as to be life-threatening, it is called a thyroid storm which can greatly increase heart rate (palpitations), BP and body temperature. It can be seen in patients with Grave’s disease.
Both myxedema crisis and thyroid storm are life-threatening situations and are considered medical emergencies.
Note – An enlarged thyroid gland may present with no symptoms at all, while a patient with symptoms of hyper or hypothyroidism may not have any enlargement or swelling of the thyroid gland!
In addition to thyroid function blood tests (TSH, T4/T3), your doctor may recommend a thyroid ultrasound or radioactive thyroid scan, and sometimes a needle aspiration of a thyroid nodule to diagnose the cause of the thyroid disorder. A significantly raised level of thyroid peroxidase (TPO) antibodies can be a useful marker for autoimmune thyroid disease. As thyroid hormones also have a stimulatory effect on Hemoglobin and Red Blood Cell (RBC) count, unexplained anemia seen in CBC (Complete Blood Count) test, can sometimes be secondary to decreased thyroid function.
Iodine deficiency used to be a common cause of goiter and hypothyroidism in the past. With the introduction of iodized salt where Iodine (in the form of Potassium Iodide) is added to common salt (15-45mg Iodine/kg salt), iodine deficiency is rarely seen. It is interesting to note that we consume around 5-10g salt per day so the amount of Iodine intake and the requirement is around 100-150 micrograms (mcg).
Intake of very large doses of Iodine (30-50mg/day) can actually inhibit or reduce the synthesis of thyroid hormones. This approach is used in patients who are hyperthyroid or have thyrotoxicosis. In some cases, radioactive Iodine may be used to suppress/shrink an overactive thyroid gland.
Medicines like methimazole/carbimazole and propylthiouracil (reserved for thyroid storm) are also used and act by inhibiting thyroid hormone synthesis. Other drugs like propranolol (beta-blockers) or diltiazem may be used to reduce palpitation and abnormally high heart rate. Surgical options may also be required especially in situations like thyroid tumors or unresponsive thyrotoxicosis.
Hypothyroid cases are treated by intake of thyroid hormone (levothyroxine – usual dose 25-50 mcg). Subclinical cases where TSH is raised but free T4/T3 is normal are treated in certain situations like pregnancy (planning or current), if clinical symptoms are present, and if there is evidence of autoimmune disease.
Note- Monitoring of thyroid function with TSH and T4/T3 is recommended regularly and treatment for thyroid disorders should be very closely medically supervised as sometimes side effects may be significant, and the slightest change in dose can cause a large change in hormonal levels.
Additional reading –
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