Thyroid Hormones

Thyroid hormones don’t get as much attention in fitness world as their more anabolic relatives, but proper thyroid function is essential for regulation of metabolic rate and body temperature, in addition to other crucial biological responsibilities. This makes the thyroid a topic worthy of further study for exercise enthusiasts — don’t underestimate the effect of thyroid function on physical performance and body composition. Thyroid hormones are responsible for the regulation of oxygen use, basal metabolic rate, and cellular metabolism and developmentBy stimulating cellular oxygen use for the production of ATP, thyroid hormones increase basal metabolic rate, which is the rate of oxygen consumption at rest. Cells use more oxygen to produce ATP — this causes body temperature to rise as more heat is given off. This maintenance process of body temperature (thermoregulation) is called the calorigenic effect.

As to the regulation of metabolism, thyroid hormones prompt protein synthesis and increase the use of glucose for production of ATP. They work together with growth hormone and insulin to accelerate tissue growth.  Secretion from the thyroid is controlled by both iodine levels and negative feedback systems involving the hypothalamus and pituitary. The negative feedback systems are triggered by factors such as environmental cold, high altitude, and hypoglycemia. The thyroid gland is the only endocrine gland that stores its product in very large quantities. Thyroid follicles fill the thyroid gland, where they manufacture Thyroxine (T4) and Triiodothyronine (T3). Normally, T4 is produced on greater amounts, but isn’t as potent as T3. These are both lipid soluble and enter the blood by diffusing into the plasma membrane. In the blood, they combine with transport proteins, particularly thyroxine-binding globulin. The majority of T4 converts into T3 when in the bloodstream, which is the more active of the two forms. For this reason, T4 functions bascially as a prohormone. Inadequate conversion of T4 to T3 can result in hypothyroidism.

I know, it’s all very science-y. Well, strap in, because we’re about to get more academic.

Here’s a study that investigates thyroid function in pro cyclists:

Thyroid hormones may influence the slow component of VO(2) in professional cyclists. Lucía A, Hoyos J, Pérez M, Chicharro JL.Departamento de Ciencias Morfológicas y Fisiología, Universidad Europea de Madrid, Spain.  http://www.ncbi.nlm.nih.gov/pubmed/11405918

“We analyzed the relationship between the plasma concentrations of several hormones (testosterone [T], follicle-stimulating [FSH] and luteinizing hormone [LH], cortisol [C], 3,5,3′-triiodothyronine [T(3)], thyroxine [T(4)], and thyrotrophin [TSH]) and the magnitude of the VO(2) slow component (Delta VO(2)) in a group of nine professional road cyclists (26+/-2 years). Plasma concentrations of T(3) and T(4) were inversely correlated (p<0.05) with Delta VO(2) (r=-0.72 and rr=-0.66, respectively), suggesting, at least partly, and association between thyroid basal function and the VO(2) slow component of euthyroid elite endurance athletes during constant-load intense exercise.”

T3 and T4 ended up being inversely correlated, which meant that they didn’t rise or fall together. Remember, T4 is basically the prohormone that converts to the T3 hormone.

The following study gives a look into cardiovascular function in patients with thyroid disease:

Differences in heart rate profile during exercise among subjects with subclinical thyroid disease. Maor E, Kivity S, Kopel E, Segev S, Sidi Y, Goldenberg I, Olchovsky D.Leviev Heart Institute, Chaim Sheba Medical Center. Oct;23(10):1226-32. doi: 10.1089/thy.2013.0043. http://www.ncbi.nlm.nih.gov/pubmed/23777550

“Clinical thyroid disease is associated with changes in the cardiovascular system, including changes in heart rate during exercise… Subjects with SCHypoT showed a trend toward a lower resting heart rate (75±13 vs. 77±15 bpm, p=0.09) and had a significantly lower recovery heart rate (88±12 vs. 90±13 bpm, p=0.035). Conclusions: Subjects with SCTD have a significantly different heart rate profile during rest, exercise, and recovery.”

The above makes clear the importance of the thyroid in relation to exercise, but can exercise affect thyroid hormone production in a similar way as it can GH and Testosterone? Well, let’s find out:

Exercise intensity and its effects on thyroid hormones. Figen Ciloglu, Ismail Peker1, Aysel Pehlivan, Kursat Karacabey, Nevin İlhan, Ozcan Saygin & Recep Ozmerdivenli. University of Gaziantep, The School of Physical Education and Sports, Gaziantep, Turkey. http://www.nel.edu/26-2005_6_pdf/NEL260605A14_Ciloglu.pdf

The results of this study show that exercise performed at the anaerobic threshold (70% of maximum heart rate, lactate level 4.59 ± 1.75 mmol/l) caused the most prominent changes in the amount of any hormone values. While the rate of T4, fT4, and TSH continued to rise at 90% of maximum heart rate, the rate of T3 and fT3 started to fall. Conclusions: Maximal aerobic exercise greatly affects the level of circulating thyroid hormones.”

To summarize, as intensity escalates, more T4 is produced, but less is converted to T3. Keep in mind that the change in homeostasis is temporary, because much like the case with GH and Testosterone levels, the circulating hormones will slowly return to pre-exertion levels after the activity stops.

Moving on, we’ll look at one more study, this one comparing thyroid hormonal response to differing exercise protocols — Interval training versus steady state cardio in highly trained test subjects.

Thyroid hormonal responses to intensive interval versus steady-state endurance exercise sessions. Hackney AC, Kallman A, Hosick KP, Rubin DA, Battaglini CL.Endocrine Section-Applied Physiology Laboratory, Department of Exercise & Sport Science, University of North Carolina, Chapel Hill, North Carolina. http://www.ncbi.nlm.nih.gov/pubmed/22450344

IE results in a suppressed peripheral conversion of T₄ to T₃ implying that a longer recovery period is necessary for hormonal levels to return to normal following IE compared to SEE. These findings are useful in the implementation of training regimens relative to recovery needs and prevention of over-reaching-overtraining.”

The verdict is that the intense interval training resulted in a longer time period before T4 to T3 conversion restarted at normal pre-exercise levels. Useful information for programming a fitness routine for someone suffering from hypothyroidism.

A healthy metabolism depends on a properly functioning thyroid. Obviously, that means body composition and fat loss are closely tied to it also. For those with hypothyroidism, exercise in combination with medical treatment can be extremely helpful in maintaining a healthy body composition and improving thyroid function over the long term.

Sources

ACSM’s American College of Sports Medicine. The recommended quantity and quality of exercise for developing and maintaining
cardiorespiratory and muscular fitness in healthy adults. Med Sci Sports Exerc 1990; 22: 265–274.

Differences in heart rate profile during exercise among subjects with subclinical thyroid disease. Maor E, Kivity S, Kopel E, Segev S, Sidi Y, Goldenberg I, Olchovsky D.Leviev Heart Institute, Chaim Sheba Medical Center. Oct;23(10):1226-32. doi: 10.1089/thy.2013.0043. http://www.ncbi.nlm.nih.gov/pubmed/23777550

Exercise intensity and its effects on thyroid hormones. Figen Ciloglu, Ismail Peker1, Aysel Pehlivan, Kursat Karacabey, Nevin İlhan, Ozcan Saygin & Recep Ozmerdivenli. University of Gaziantep, The School of Physical Education and Sports, Gaziantep, Turkey.  http://www.nel.edu/26-2005_6_pdf/NEL260605A14_Ciloglu.pdf

Hackney AC, McMurray RG, Judelson DA, Harrell JS.. Relationship between caloric intake, body composition, and physical activity
to leptin, thyroid hormones, and cortisol in adolescents. Jpn. J Physiol. 2003; 53:475–9.

Huang WS, Yu MD, Lee MS, Cheng CY, Yang SP, Chin HM, Wu SY. Effect of treadmill exercise on circulating thyroid hormone measurements. Med Princ Pract. 2004;13:15–9.

Lucia A, Hoyos J, Perez M, Chicharro JL.Thyroid Hormones may influence the slow component of VO2 in professional cyclists. Japanese Journal of Physiology 2001; 51:239–242.

Thyroid hormonal responses to intensive interval versus steady-state endurance exercise sessions. Hackney AC, Kallman A, Hosick KP, Rubin DA, Battaglini CL.Endocrine Section-Applied Physiology Laboratory, Department of Exercise & Sport Science, University of North Carolina, Chapel Hill, North Carolina. http://www.ncbi.nlm.nih.gov/pubmed/22450344

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