We have much to learn from older physiology textbooks. They are often a source of invaluable knowledge which has been lost/abandoned over the last decades of the 20th century.
A book such as McGavack’s The Thyroid, that provides traditional medical knowledge about thyroid physiology, can help to dispel some of the current dogmas about the thyroid.– Ray Peat (TSH, temperature, pulse rate, and other indicators in hypothyroidism)
(In this short article, I pulled a few quotes regarding vitamin A from the aforementioned book.)
The relationship between Vitamin A and Thyroid
Vitamin A seems to have a very close, if nonlinear, relationship with thyroid function. When deficient, it will limit thyroid function, and in excess, it will suppress it. In the western world, vitamin A deficiencies are quite rare, in part because it is ubiquitous in our food supply, but also I think because the vast majority of the population has become hypothyroid over the last decades.
Quotes from McGavack (1951):
In experimental or clinical conditions associated with an increase in the amount of thyroid hormone, as, for instance, in thyrotoxicosis, there is simultaneously an increased need for vitamin A (Sure and Buchanan, 1937; Logaras and Drummond; 1938; Wegelin, 1939; Remington, Harris, and Smith, 1942; Mandelbaum, Candel, and Millman, 1942; Interrelations between thyroid function and vitamin metabolism, 1944).
Conversely, thyroidectomized animals apparently need less vitamin A than normal animals (Remington, Harris, and Smith, 1942; Drill, 1943; Wiese, Mehl, and Deuel, 1948).
A lack of vitamin A initially produces in experimental animals hypertrophy of the thyroid (Lipsett and Winzler, 1947), which if sufficiently long continued gives way to an atrophy that is usually more marked in the male than in the female.
In some respects the action of vitamin A and the thyroid hormone are mutually antagonistic. If vitamin A and thyroid substance are administered simultaneously, the ability of the thyroid hormone to raise the basal metabolic rate is markedly decreased.
…in hyperthyroidism there is a definite increase in the requirements for vitamins A, B, and C, which, if not met, will result in a relative deficiency.
Vitamin A seems to play fundamental roles, notably in steroidogenesis, and as thyroid function increases, so do the requirements of an individual for vitamin A and other nutrients. It’s quite possible that nowadays most people don’t require more vitamin A than they already get, in part since their requirements are probably very low because of their sluggish thyroid.
In hypothyroid states and in thyroidectomized animals the ability of the liver to convert carotene to vitamin A is decreased or lost (Drill and Truant, 1947a, b; Kelley and Day, 1948), and the capacity of the liver for the storage and mobilization of this vitamin is disturbed.
The well-known ability of thyroxin to mobilize glycogen from within the liver can be modified of even checked completely by the use of vitamin A. Wegelin (1939) believes that this is due to a storing effect of vitamin A.
In summary, it would seem that hypothyroidism and thyroidectomy interfere with the conversion of carotene to vitamin A, decrease storage in the liver, and reduce the requirements for this vitamin, but do not interfere with its utilization. On the other hand, when there is an excess of thyroid hormone, as in thyrotoxicosis, the requirements for vitamin A is increased (…)
A big meal of beef liver (which contains A LOT of retinol) can suppress thyroid function, but at the same time it can probably have a synergistic effect if the requirements of the individual eating it are elevated. So context matters greatly. A glass of fresh carrot juice can be energizing and replenishing for someone with optimal thyroid function, but it can further suppress metabolism in many people. If a low thyroid state is maintained for years against a western diet, there is the possibility that vitamin A could accumulate in the liver and tissues and cause problems. The overarching idea is that nuance and context are important. I think vitamin A is absolutely essential for truly optimal human health, but it is probably necessary to think of it in relation to thyroid function.