St. Petersburg Health & Wellness

Thyroid Function – Les Cole, MD
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Thyroid Gland

How Do The Thyroid Gland and Thyroid Hormones Work?

Your Thyroid is a gland located in the front lower portion of your neck. It has two lobes just below and on either side of your larynx or Adam’s apple and are connected by a small portion of gland called the isthmus. This is where your thyroid hormone is produced.

Two hormones are produced. Thyroxine, also called T4, is made of 2 Tyrosine molecules to which 4 iodine molecules are attached. Triiodothyronine, also called T3, because it is 2 Tyrosines with 3 iodines.

Adequate levels of both Tyrosine (an amino acid from protein digestion) and Iodine are needed to make these hormones. In addition, you need the following nutrients for your Thyroid gland to produce thyroid hormone: Zinc, Copper, Vitamines A, B2, B3, B6, and C.

Your thyroid gland produces mostly T4 (which is the pro-hormone to T3) and a small amount of T3. These are secreted by your gland into your blood stream where most of the hormone is bound to protein – Thyroid Binding Globulin, transthyretin & albumin. While bound, T3 is not available to your tissues and T4 is not available for conversion to T3. A small amount circulates in the blood unbound as free T3 (fT3) and free T4 (fT4) which are available to your tissues and in the case of T4 for conversion. The bound hormone acts as a ready reservoir for free hormone, so that, as your free hormone gets used, more of the bound hormone is freed up for use.

Control of Thyroid Secretion

Thyroid secretion occurs under the control of Thyroid Stimulating Hormone (TSH aka Thyrotropin) which stimulates your thyroid gland to produce more hormone. TSH comes from your pituitary – an area of your brain that secretes many control hormones affecting most of the glands in your body. What causes TSH to be released? TSH release is controlled by specific areas of your hypothalamus – a portion of your brain that controls many functions of your body. This area has Thyroid receptors on it so it knows how much circulating Thyroid Hormone you have in your blood. Thyrotropin Releasing Hormone (TRH) is released by your hypothalamus when it senses low levels. Higher levels of circulating hormone join with the receptors in your hypothalamus and this tells your hypothalamus that you have “enough” thyroid hormone, which causes TRH release to be shut off. This in turn shuts off the TSH.

The Thyroid Hormones T4 and T3

  • T4 is primarily a pro-hormone and T3 is the active hormone.
  • T4 has little physiologic activity
  • T4 must be converted to T3 to be utilized by cells – with a few exceptions
  • T3 has 5 times the potency T4
  • T3 has 5 times the distribution volume of T4 throughout the body
  • T3 has direct effects on the heart that T4 does not
    • Klein, I, et al. Thyroid hormone & the cardiovascular system. NEMJ 2002;244:501-509
  • T3 has direct effects on the mitochondria that T4 does not
    • Wrutniak-Cabello, C, et al. Thyroid hormone action in mitochondria. J Mol Endocrin 2001;26:67-77
  • Intracellular thyroid hormone receptors have a high affinity for T3 – not T4
  • 90% of thyroid hormone molecules that bind receptors are T3 & only 10% are T4

Conversion of T4 to T3 occurs by way of Deiodinase enzymes. As you can see from above, the conversion of T4 to T3 is crucial to your cells to maintain healthy metabolism. You have 3 Deiodinase enzymes – Deiodinase 1, 2 & 3 – and they require selenium and iron to work, as well as, zinc and vitamins A, B2, B6 & B12. These perform many functions such as conserving iodine from the breakdown of your thyroid hormone, control of thyroid production and conversion of your T4 to T3. Deiodinase 1 & 2 (D1 & D2) cause your T4 to be converted to active T3 and so are very important to maintaining your metabolism. These are located in your thyroid gland, hypothalamus, liver, kidney and throughout your body. In your thyroid, they are responsible for the small amount of T3 that your gland secretes. In your tissues, they allow activation to T3 to produce and maintain your mitochondria for energy production and temperature maintenance and a host of other functions. In the hypothalamus, they are involved in feedback control of your thyroid hormone production. D3 inactivates T4 by converting it to reverse T3 (rT3), which then competes with the active form of T3 at your T3 receptors. rT3 can be measured and is one of the indicators of how your thyroid is functioning. This is a very simplified discussion of Deiodinase.

Factors that inhibit the conversion of T4 to T3:

  • Nutritional deficiencies
    • Iodine
    • Iron
    • Selenium
    • Zinc
    • Vitamins A, B2, B6, B12
  • Medications
    • Beta Blockers – block both sympathetic system & thyroid (lethargy & wt gain)
    • Estrogen – Oral Estrogen replacement depletes the body of B vitamins
    • Birth Control Pills
    • Lithium
    • Phenytoin
    • Theophylline
    • Chemotherapy
    • Clomipramine
    • Glucocorticoids
    • Interleukin 6 – either given as a drug or elevated from inflammation.
  • Diet
    • Cruciferous vegetables in excess
    • Low protein, low fat or low carbohydrate diet
    • High carbohydrate diet
    • Excess alcohol use – (B1 – thiamine deficiency)
    • Walnuts in excess
    • Soy
  • Other
    • Stress
    • Elevated cortisol
    • Low cortisol, DHEA
    • Starvation
    • Chronic illness
    • Decreased kidney or liver function
    • Inflammation
    • Aging
    • Diabetes
    • Surgery – stress
  • Supplement excess
    • Calcium excess (1600 mg max – total/d postmenopausally)
    • Copper excess (interferes w/ Zinc)
    • Alpha Lipoic Acid excess (600-1800 mg)
  • Toxins
    • Floride
    • Lead
    • Mercury
    • Phtalates (can measure in body)
    • BPA
    • PCB
    • Pesticides
    • Dioxins
    • Radiation


What Your Thyroid Hormones Do For You – Functions of the Thyroid Hormones:

(Remember, it is mostly your T3 that provides these benefits that keep you healthy)

  • Act on every cell in your body
  • Increases the number, function and health of your mitochondria – the power plants and furnaces of your cells – one of T3s most important functions
    • As power plants, your mitochondria produce adenosine tri-phosphate (ATP)
      • ATP is your “energy currency”
        • It is the only way things get done in your body
        • It is the source of energy for all anabolism (building) in your body
    • As furnaces, your mitochondria produce the heat to maintain your body temperature at 98.6 degrees Fahrenheit
      • This temperature is critical for the optimal functioning of your body
  • Effects your Basal Metabolic Rate (BMR) or the amount of energy (fat & sugar) you burn at rest
    • More & better functioning mitochondria mean more energy burnt
  • Maintains your energy at active level
    • By increasing the amount of fat & sugar you burn in your mitochondria to increase your “energy-currency”
    • By increasing the number of mitochondria you produce more “energy currency”
  • Maintains your weight at healthy levels
    • With more & healthier mitochondria you burn more fat & sugar
    • Eating sugar (& carbs) puts more fat on you than fat does – this is why you lose weight on low-carb diets
      • All carbs that you eat except fiber are absorbed as sugar – so watch your carbs!
      • All sugar that doesn’t get burned for energy goes to the liver to be converted to fat and then shipped to fat cells for storage
    • Muscle burns mostly fat at rest – so build muscle through exercise!
    • With more energy, you feel like exercising more!
  • Maintains your blood sugar at healthy levels
    • Hyperglycemia occurs when you don’t burn as much sugar as you eat
    • As your blood sugar rises, insulin increases leading to insulin resistance (IR) & type 2 diabetes (T2DM)
    • As you burn more sugar, your blood sugar is easier to control – especially with today’s diets and lack of exercise
  • Maintains your blood lipids (Cholesterol & Triglycerides) at healthy levels
    • Before thyroid hormones could be measured, serum cholesterol levels used to be measured to determine if you were hypothyroid
      • If elevated, you were diagnosed as hypothyroid and treated with thyroid hormone, which then lowered them back toward normal
  • Maintains your body temperature at healthy levels
    • Your body works best at 98.6
    • As fat & sugar are burnt in your mitochondria, most of the energy released is captured as ATP “energy currency” and some escapes as heat – important for both power plant & furnace functions of your mitochondria
    • “Uncoupling protein” increases the amount of heat produced
    • T3 stimulates production of “uncoupling protein” in your mitochondria
    • Along with increasing the number of your mitochondria, this is how T3 helps maintain your body temperature
      • “Uncoupling protein” also helps maintain your ideal weight by increasing the burning of fat & sugar
    • This is why if your resting body temperature is low, you are likely hypothyroid and feel intolerant to cold and have cold hands & feet
    • Also by promoting normal sweat gland function in your skin, T3 allows you to perspire, evaporate the sweat and cool your body down, which is why patients with hypothyroidism are also heat intolerant
  • Promotes healthy protein synthesis
  • Stimulates healthy vitamin metabolism
  • Improves functioning of your sympathetic nervous system
  • Is ultimately important in the growth and normal development of children from conception to adulthood
  • Improves the “clock speed” of your brain
    • “Clock speed” is how fast things happen in your cells
    • In your brain this is everything and affects the speed of your:
      • Cognition or thinking
      • Memory
      • Awareness
      • Concentration or focus
      • Learning
  • Maintains your brains healthy neurotransmitter levels
  • Promotes your emotional health
  • Promotes healthy myelination, blood vessels, neuronal networks and neurogenesis (the growth of new neurons from brain stem cells) in your brain
  • By promoting the health of your immune system, your thyroid hormones help optimize your immune systems function
    • Preventing hypofunction & thus improving:
      • Infection surveillance – preventing infections
      • Cancer surveillance – in a recent study of subjects with “normal” T3 levels (in other words, not considered hypothyroid), those in the lowest quartile had a significantly increased risk of dying from cancer than did those with T3 levels in the highest quartile! (Zhang Y, et. al. Thyroid hormones and mortality risk in euthyroid individuals: The Kangbuk Samsung Health Study. J Clin Endocrinol Metab. 2014 Apr 7:jc20133832. [Epub ahead of print])
    • Preventing hyperfunction
      • Asthma
      • Allergies
      • Hives
  • Maintains your cardiovascular system in a healthy state
    • By lowering your cholesterol & triglyceride levels
    • By maintaining the “clock speed” of the heart conduction system and assisting the sympathetic nervous system, your T3
      • Prevents hypotension (low blood pressure)
      • Helps prevent postural hypotension (feeling dizzy when stand)
      • Prevents bradycardia (slowed heart rate)
      • Prevents some forms of tachycardia & arrhythmia (fast or irregular heart rate)
      • Maintains your heart rate variability (HRV) – very important to your heart health
    • By improving your heart’s contractility (strength of contraction), your T3 keeps your heart strong and prevents heart failure
    • By increasing the dilation of your blood vessels known as your peripheral resistance arterioles, T3
      • Prevents hypertension (high blood pressure)
  • Maintains the health of your muscles (including heart) and connective tissue promoting comfort and preventing you from having
    • Muscle aches and pains
    • Muscle weakness
    • Muscle cramps (especially calf)
    • Repetitive tendon injuries
  • Your T3 increases the lubrication of your joints helping them to feel comfortable and stay healthy
  • By maintaining several of your biological systems, your thyroid helps promote a healthy appearance
    • Your T3 promotes the conversion of carotene to vitamin A, which helps your appearance in 2 ways
      • A build-up of unconverted carotene in your skin will make you look like John Boehner…Just kidding…It causes your skin to turn yellow to orange
      • Vitamin A is critical to healthy skin and therefor your complexion
    • By preventing accumulation of excess glycoprotein in your tissues T3 prevents a puffy or swollen appearance (and snoring with its risk of sleep apnea)
    • T3 maintains the function of the glands in your skin helping it remain blemish free – sluggish glands promote blackheads & acne
      • Also by promoting normal sweat gland function in your skin, T3 allows you to perspire, evaporate the sweat and cool your body down, which is why patients with hypothyroidism are also heat intolerant
    • Your hair requires adequate levels of T3 for its health – maintaining lustrous healthy growth of the hair on your head, eyebrows, eyelashes and elsewhere on your body
    • T3 promotes the health and strength of your finger and toe nails
  • Your T3 also helps you sleep and feel refreshed on waking in the morning
    • By preventing swelling of your airways by excess glycoprotein, it may prevent snoring & sleep apnea – a common cause of sleep disruption, low oxygen levels while sleeping & a whole host of consequent health issues
    • Thyroid provides the energy to feel refreshed in the morning
  • T3 maintains adequate oxygen (O2) delivery to your tissues keeping them healthy
    • By improved heart function and cardiac output
    • Improved respiration
    • Increased erythropoietin hormone levels produced by the kidney, which go to the bone marrow and increase the production of red blood cells (RBCs) which can carry more O2
  • Your oxygen utilization is improved by improving the health of your mitochondria
  • Oxidative stress is reduced when your oxygen utilization is improved and with healthier mitochondria
  • Your thyroid hormone contributes to an active libido (interest in sex), along with many other hormones
  • Your T3 (along with testosterone) is important for achieving and maintaining a healthy erection
    • Your T3 level can have a dramatic effect on your orgasms If it is too low, it can make them difficult to achieve, prolong their onset, diminish their intensity or even make them non-existent
    • If it is too high, it can make them too quick (premature ejaculation) or make them excessively intense
  • Your ability to reproduce is greatly affected by your thyroid hormone
    • As a male, the speed & energy of your sperm are extremely dependent on how much “energy currency” they carry. Without sufficient T3, they can’t go the distance.
    • As a woman, your hormonal symphony is very important and can be disrupted, preventing ovulation
  • Your ability to carry a baby to full term is supported by your thyroid hormone
    • When it is low or if you have anti-thyroid antibodies (as in Hashimoto’s Thyroiditis) your risk of miscarriage is increased
  • Adequate T3 is important also for the optimal development of your baby’s brain and IQ during your pregnancy
  • Part of the hormonal symphony that T3 regulates is prolactin & oxytocin levels, which enable you to breast feed your new baby. There is even evidence that thyroid levels in the low normal range can decrease your ability to produce milk
  • Your ear health, hearing and balance are maintained by your thyroid hormones
    • Ringing in your ears, dizziness and loss of hearing in old age (presbycusis) can all be symptoms of low thyroid
    • As are itchy ears and excessive wax production
  • Your neurologic health is dependent on your thyroid hormones
    • Headache/Migraines, RLS and Carpel Tunnel Syndrome are among the many symptoms of low thyroid
    • Thinking, concentration, focus, clear-headedness all require adequate thyroid hormone


To see a list of many of the symptoms of hypothyroidism go to: Signs & Symptoms of Hypothyroidism. To see if you may be hypothyroid download, complete, save and email it back to [email protected]. If you have any questions or to make an appointment call 727-202-6807. If you are unable to fill this form because you have Word 2007 or before try downloading the Microsoft Office Compatibility Pack for Word, Excel and PowerPoint Open XML File Formats.

Additional information on thyroid may be found on this site: Signs & Symptoms of Hypothyroidism, Signs & Symptoms of Hyperthyroidism, Thyroid Testing, Am I Hypothyroid?

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